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FEATURES
■Dual N-Channel MOSFET Synchronous Drive
■Programmable Fixed Frequency
■Wide VIN Range: 3.5V to 36V Operation
■Ultrahigh Efficiency
■Very Low Dropout Operation: 99% Duty Cycle
■Low Standby Current
■Secondary Feedback Control
■Programmable Soft Start
■Remote Output Voltage Sense
■Logic Controlled Micropower Shutdown: IQ < 25μA
■Foldback Current Limiting (Optional)
■Current Mode Operation for Excellent Line and Load Transient Response
■Output Voltages from 1.19V to 9V
■Available in 16-Lead Narrow SO and SSOP Packages
APPLICATIOUS
■Notebook and Palmtop Computers, PDAs
■Cellular Telephones and Wireless Modems
■Portable Instruments
■Battery-Operated Devices
■DC Power Distribution Systems
LTC1435
High Efficiency Low Noise
Synchronous Step-Down
Switching Regulator
DESCRIPTIOU
The LTC®1435 is a synchronous step-down switching regulator controller that drives external N-channel power MOSFETs using a fixed frequency architecture. Burst ModeTM operation provides high efficiency at low load currents. A maximum duty cycle limit of 99% provides low dropout operation which extends operating time in bat- tery-operated systems.
The operating frequency is set by an external capacitor allowing maximum flexibility in optimizing efficiency. A secondary winding feedback control pin, SFB, guarantees regulation regardless of load on the main output by forcing continuous operation. Burst Mode operation is inhibited when the SFB pin is pulled low which reduces noise and RF interference.
Soft start is provided by an external capacitor which can be used to properly sequence supplies. The operating current level is user-programmable via an external current sense resistor. Wide input supply range allows operation from 3.5V to 30V (36V maximum).
, LTC and LT are registered trademarks of Linear Technology Corporation.
Burst Mode is a trademark of Linear Technology Corporation.
TYPICAL APPLICATIOU
VIN
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4.5V TO 28V |
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COSC |
COSC |
VIN |
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CIN |
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M1 |
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68pF |
RUN/SS |
TG |
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22 F |
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Si4412DY |
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35V |
RSENSE |
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CSS |
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0.1 F |
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× 2 |
0.033Ω |
VOUT |
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SW |
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CC |
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DB |
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L1 |
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2.9V/3.5A |
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LTC1435 |
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330pF |
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CMDSH-3 |
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10 H |
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R1 |
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RC |
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INTVCC |
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CB |
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+ COUT |
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32.4k |
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10k |
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0.1 F |
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100 F |
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SGND |
BOOST |
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R2 |
10V |
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D1 |
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× 2 |
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100pF |
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M2 |
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22.1k |
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VOSENSE |
BG |
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4.7 F |
MBRS140T3 |
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Si4412DY |
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PGND |
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SENSE– |
SENSE+ |
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1000pF |
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1435 F01 |
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Figure 1. High Efficiency Step-Down Converter
1
LTC1435
ABSOLUTE WAXIWUW RATINGSU
Input Supply Voltage (VIN)......................... |
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36V to – 0.3V |
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Topside Driver Supply Voltage (Boost) ...... |
42V to – 0.3V |
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Switch Voltage (SW)............................. |
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VIN + 5V to – 5V |
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EXTVCC Voltage ........................................ |
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10V to – 0.3V |
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Sense+, Sense– Voltages ......... |
INTVCC + 0.3V to – 0.3V |
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ITH, VOSENSE Voltages .............................. |
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2.7V to – 0.3V |
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SFB, Run/SS Voltages .............................. |
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10V to – 0.3V |
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Peak Driver Output Current < 10μs (TG, BG) |
............. 2A |
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INTVCC Output Current ........................................ |
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50mA |
Operating Ambient Temperature Range |
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0°C to 70°C |
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LTC1435C............................................... |
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LTC1435I............................................ |
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– 40°C to 85°C |
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Junction Temperature (Note 1)............................. |
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125°C |
Storage Temperature Range ................. |
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– 65°C to 150°C |
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Lead Temperature (Soldering, 10 sec).................. |
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300°C |
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PACKAGE/ORDER IUFORWATIOU
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TOP VIEW |
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ORDER PART |
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NUMBER |
COSC |
1 |
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16 |
TG |
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RUN/SS |
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2 |
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15 |
BOOST |
LTC1435CG |
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ITH |
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3 |
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14 |
SW |
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LTC1435CS |
SFB |
4 |
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13 |
VIN |
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LTC1435IG |
SGND |
5 |
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12 |
INTVCC |
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LTC1435IS |
VOSENSE |
6 |
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11 |
BG |
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SENSE– |
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10 |
PGND |
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SENSE+ |
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8 |
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9 |
EXTVCC |
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G PACKAGE |
S PACKAGE |
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16-LEAD PLASTIC SSOP 16-LEAD PLASTIC SO |
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TJMAX = 125°C, θJA = 130°C/ W (G) |
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TJMAX = 125°C, θJA = 110°C/ W (S) |
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Consult factory for Military grade parts.
ELECTRICAL CHARACTERISTICS TA = 25°C, VIN = 15V, VRUN/SS = 5V unless otherwise noted.
SYMBOL |
PARAMETER |
CONDITIONS |
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TYP |
MAX |
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UNITS |
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Main Control |
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Loop |
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IIN VOSENSE |
Feedback Current |
(Note 2) |
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10 |
50 |
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nA |
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VOSENSE |
Feedback Voltage |
(Note 2) |
● |
1.178 |
1.19 |
1.202 |
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V |
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VLINEREG |
Reference Voltage Line Regulation |
VIN = 3.6V to 20V (Note 2) |
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0.002 |
0.01 |
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%/V |
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VLOADREG |
Output Voltage Load Regulation |
ITH Sinking 5μA (Note 2) |
● |
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0.5 |
0.8 |
% |
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ITH Sourcing 5μA |
● |
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– 0.5 |
– 0.8 |
% |
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VSFB |
Secondary Feedback Threshold |
VSFB Ramping Negative |
● |
1.16 |
1.19 |
1.22 |
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V |
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ISFB |
Secondary Feedback Current |
VSFB = 1.5V |
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– 1 |
– 2 |
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μA |
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VOVL |
Output Overvoltage Lockout |
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1.24 |
1.28 |
1.32 |
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V |
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IQ |
Input DC Supply Current |
EXTVCC = 5V (Note 3) |
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μA |
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Normal Mode |
3.6V < VIN < 30V |
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260 |
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Shutdown |
VRUN/SS = 0V, 3.6V < VIN < 15V |
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16 |
25 |
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μA |
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VRUN/SS |
Run Pin Threshold |
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● |
0.8 |
1.3 |
2 |
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V |
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IRUN/SS |
Soft Start Current Source |
VRUN/SS = 0V |
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1.5 |
3 |
4.5 |
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μA |
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VSENSE(MAX) |
Maximum Current Sense Threshold |
VOSENSE = 0V, 5V |
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130 |
150 |
180 |
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mV |
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TG Transition Time |
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TG tr |
Rise Time |
CLOAD = 3000pF |
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50 |
150 |
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ns |
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TG tf |
Fall Time |
CLOAD = 3000pF |
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50 |
150 |
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ns |
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BG Transition Time |
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BG tr |
Rise Time |
CLOAD = 3000pF |
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50 |
150 |
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ns |
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BG tf |
Fall Time |
CLOAD = 3000pF |
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40 |
150 |
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ns |
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Internal VCC |
Regulator |
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VINTVCC |
Internal VCC Voltage |
6V < VIN < 30V, VEXTVCC = 4V |
● |
4.8 |
5.0 |
5.2 |
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V |
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VLDO INT |
INTVCC Load Regulation |
IINTVCC = 15mA, VEXTVCC = 4V |
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– 0.2 |
– 1 |
% |
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VLDO EXT |
EXTVCC Voltage Drop |
IINTVCC = 15mA, VEXTVCC = 5V |
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130 |
230 |
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mV |
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VEXTVCC |
EXTVCC Switchover Voltage |
IINTVCC = 15mA, VEXTVCC Ramping Positive |
● |
4.5 |
4.7 |
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V |
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Oscillator |
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fOSC |
Oscillator Frequency |
COSC = 100pF (Note 4) |
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112 |
125 |
138 |
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kHz |
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2
LTC1435
ELECTRICAL CHARACTERISTICS TA = 25°C, VIN = 15V, VRUN/SS = 5V unless otherwise noted.
The ● denotes specifications which apply over the full operating temperature range.
LTC1435CG/LTC1435CS: 0°C ≤ TA ≤ 70°C
LTC1435IG/LTC1435IS: – 40°C ≤ TA ≤ 85°C
Note 1: TJ is calculated from the ambient temperature TA and power dissipation PD according to the following formula:
LTC1435CG/LTC1435IG: TJ = TA + (PD)(130°C/W)
LTC1435CS/LTC1435IS: TJ = TA + (PD)(110°C/W)
Note 2: The LTC1435 is tested in a feedback loop which servos VOSENSE to the balance point for the error amplifier (VITH = 1.19V).
Note 3: Dynamic supply current is higher due to the gate charge being delivered at the switching frequency. See Applications Information. Note 4: Oscillator frequency is tested by measuring the COSC charge and discharge currents and applying the formula:
fOSC (kHz) = ( |
8.4(108) |
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)–1 |
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IDIS |
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COSC (pF) + 11 ICHG |
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TYPICAL PERFORWAUCE CHARACTERISTICS
Efficiency vs Input Voltage
VOUT = 3.3V
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VOUT |
= |
3.3V |
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ILOAD |
= 1A |
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(%) |
90 |
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EFFICIENCY |
85 |
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I |
LOAD = 100mA |
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INPUT VOLTAGE (V) |
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1435 G01 |
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VIN – VOUT Dropout Voltage |
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vs Load Current |
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RSENSE = 0.033 |
Ω |
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VOUT DROP OF |
5% |
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0.4 |
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0.3 |
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OUT |
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– V |
0.2 |
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IN |
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0.1 |
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0.5 |
1.0 |
1.5 |
2.0 |
2.5 |
3.0 |
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LOAD CURRENT (A)
Efficiency vs Input Voltage
VOUT = 5V
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VOUT |
= 5V |
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ILOAD |
= 1A |
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(%) |
90 |
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I |
LOAD = 100mA |
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EFFICIENCY |
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85 |
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70 |
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5 |
10 |
15 |
20 |
25 |
30 |
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0 |
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INPUT VOLTAGE (V)
1435 G02
Load Regulation
0 |
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RSENSE = 0.033 |
Ω |
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– 0.25 |
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– 0.50 |
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(%) |
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OUT |
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– 0.75 |
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V |
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–1.00 |
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–1.25 |
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–1.50 |
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0 |
0.5 |
1.0 |
1.5 |
2.0 |
2.5 |
3.0 |
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LOAD CURRENT (A)
1435 G04 |
1435 G05 |
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Efficiency vs Load Current |
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100 |
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95 |
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VIN = 10V |
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VOUT = 5V |
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90 |
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RSENSE = 0.033Ω |
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(%) |
85 |
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80 |
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EFFICIENCY |
75 |
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CONTINUOUS |
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MODE |
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70 |
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Burst Mode |
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OPERATION |
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65 |
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60 |
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55 |
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50 |
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0.001 0.01 0.1 1 10 LOAD CURRENT (A)
1435 G03
VITH Pin Voltage vs Output Current
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3.0 |
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2.5 |
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(V) |
2.0 |
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1.5 |
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ITH |
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Burst Mode |
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V |
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1.0 |
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OPERATION |
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CONTINUOUS |
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MODE |
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0.5 |
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0
0 10 20 30 40 50 60 70 80 90 100 OUTPUT CURRENT (%)
1435 G06
3
LTC1435
TYPICAL PERFORWAUCE CHARACTERISTICS
SUPPLY CURRENT (mA)
Input Supply and Shutdown Current vs Input Voltage
2.5
2.0
1.5VOUT = 5V EXTVCC = VOUT
1.0VOUT = 3.3V
EXTVCC = OPEN
0.5
SHUTDOWN
0
0 |
5 |
10 |
15 |
20 |
INPUT VOLTAGE (V)
100 |
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80 |
SHUTDOWN |
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60 |
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A)(CURRENTμ |
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40 |
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20
0
25 30
1435 G07
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INTVCC Regulation |
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EXTVCC Switch Drop |
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vs INTVCC Load Current |
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vs INTVCC Load Current |
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0.5 |
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200 |
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VEXTVCC = |
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0V |
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180 |
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0.3 |
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(mV) |
160 |
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70°C |
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(%) |
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140 |
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CC |
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70°C |
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120 |
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25 |
°C |
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CC |
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INTV |
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0 |
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100 |
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INTV |
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– 55°C |
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CC |
80 |
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– |
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25°C |
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EXTV |
60 |
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– 0.3 |
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40 |
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20 |
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– 0.5 |
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0 |
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0 |
5 |
10 |
15 |
20 |
0 |
2 |
4 |
6 |
8 |
10 |
12 |
14 |
16 |
18 |
20 |
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INTVCC LOAD CURRENT (mA) |
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INTVCC LOAD CURRENT (mA) |
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1435 G08 |
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1435 G09 |
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FREQUENCY (%)
Normalized Oscillator Frequency vs Temperature
10
5
fO
–5
–10 |
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60 |
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110 135 |
– 40 –15 |
10 |
35 |
85 |
TEMPERATURE (°C)
1435 G10
RUN/SS CURRENT (μA)
RUN/SS Pin Current vs Temperature
4
3
2
1
0 |
35 |
60 |
85 110 135 |
– 40 –15 10 |
TEMPERATURE (°C)
1435 G11
SFB Pin Current vs Temperature
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0 |
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μA) |
– 0.25 |
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– 0.50 |
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( |
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CURRENTSFB |
–1.00 |
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– 0.75 |
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–1.25 |
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–1.50 |
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60 |
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110 135 |
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– 40 –15 |
10 |
35 |
85 |
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TEMPERATURE (°C)
1435 G12
Maximum Current Sense |
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Threshold Voltage vs Temperature |
Transient Response |
Transient Response |
CURRENT SENSE THRESHOLD (mV)
154
152
VOUT |
VOUT |
50mV/DIV |
50mV/DIV |
150
148
ILOAD = 50mA to 1A |
1435 G14 |
ILOAD = 1A to 3A |
1435 G15 |
146 |
35 |
60 |
85 110 135 |
– 40 –15 10 |
TEMPERATURE (°C)
1435 G13
4
LTC1435
TYPICAL PERFORWAUCE CHARACTERISTICS
Burst Mode Operation |
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Soft Start: Load Current vs Time |
VOUT |
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RUN/SS |
20mV/DIV |
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5V/DIV |
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INDUCTOR |
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CURRENT |
VITH |
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1A/DIV |
200mV/DIV |
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ILOAD = 50mA |
1435 G16 |
1435 G17 |
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PIU FUUCTIOUS
COSC (Pin 1): External capacitor COSC from this pin to ground sets the operating frequency.
RUN/SS (Pin 2): Combination of Soft Start and Run Control Inputs. A capacitor to ground at this pin sets the ramp time to full current output. The time is approximately 0.5s/mF. Forcing this pin below 1.3V causes the device to be shut down. In shutdown all functions are disabled.
ITH (Pin 3): Error Amplifier Compensation Point. The current comparator threshold increases with this control voltage. Nominal voltage range for this pin is 0V to 2.5V.
SFB (Pin 4): Secondary Winding Feedback Input. Normally connected to a feedback resistive divider from the secondary winding. This pin should be tied to: ground to force continuous operation; INTVCC in applications that don’t use a secondary winding; and a resistive divider from the output in applications using a secondary winding.
SGND (Pin 5): Small-Signal Ground. Must be routed separately from other grounds to the (–) terminal of COUT.
VOSENSE (Pin 6): Receives the feedback voltage from an external resistive divider across the output.
SENSE – (Pin 7): The (–) Input to the Current Comparator.
SENSE + (Pin 8): The (+) Input to the Current Comparator. Built-in offsets between SENSE– and SENSE+ pins in conjunction with RSENSE set the current trip thresholds.
EXTVCC (Pin 9): Input to the Internal Switch Connected to INTVCC. This switch closes and supplies VCC power when-
ever EXTVCC is higher than 4.7V. See EXTVCC connection in Applications Information section. Do not exceed 10V on this pin. Connect to VOUT if VOUT ³ 5V.
PGND (Pin 10): Driver Power Ground. Connects to source of bottom N-channel MOSFET and the (–) terminal of CIN.
BG (Pin 11): High Current Gate Drive for Bottom N-Channel MOSFET. Voltage swing at this pin is from ground to INTVCC.
INTVCC (Pin 12): Output of the Internal 5V Regulator and EXTVCC Switch. The driver and control circuits are powered from this voltage. Must be closely decoupled to power ground with a minimum of 2.2mF tantalum or electrolytic capacitor.
VIN (Pin 13): Main Supply Pin. Must be closely decoupled to the IC’s signal ground pin.
SW (Pin 14): Switch Node Connection to Inductor. Voltage swing at this pin is from a Schottky diode (external) voltage drop below ground to VIN.
BOOST (Pin 15): Supply to Topside Floating Driver. The bootstrap capacitor is returned to this pin. Voltage swing at this pin is from INTVCC to VIN + INTVCC.
TG (Pin 16): High Current Gate Drive for Top N-Channel MOSFET. This is the output of a floating driver with a voltage swing equal to INTVCC superimposed on the switch node voltage SW.
5