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LTC3703

3703fc

Typical applicaTion

FeaTures

applicaTions

DescripTion

100V Synchronous

Switching Regulator

Controller

The LT C

3703 is a synchronous step-down switching

regulator controller that can directly step down voltages

from up to 100V, making it ideal for telecom and automo-

tive applications. The LTC3703 drives external N-channel

MOSFETs using a constant frequency (up to 600kHz),

voltage mode architecture.

A precise internal reference provides 1% DC accuracy.

A high bandwidth error amplifier and patented line feed-

forward compensation provide very fast line and load

transient response. Strong 1Ω gate drivers allow the

LTC3703 to drive multiple MOSFETs for higher current ap-

plications. The operating frequency is user programmable

from 100kHz to 600kHz and can also be synchronized to

an external clock for noise-sensitive applications. Cur-

rent limit is programmable with an external resistor and

utilizes the voltage drop across the synchronous MOSFET

to eliminate the need for a current sense resistor. For ap-

plications requiring up to 60V operation with logic-level

MOSFETS, refer to the LTC3703-5 data sheet.

High Voltage Operation: Up to 100V

Large 1Ω Gate Drivers

No Current Sense Resistor Required

Step-Up or Step-Down DC/DC Converter

Dual N-Channel MOSFET Synchronous Drive

Excellent Line and Load Transient Response

Programmable Constant Frequency: 100kHz to

600kHz

±1% Reference Accuracy

Synchronizable up to 600kHz

Selectable Pulse-Skip Mode Operation

Low Shutdown Current: 50µA Typ

Programmable Current Limit

Undervoltage Lockout

Programmable Soft-Start

16-Pin Narrow SSOP and 28-Pin SSOP Packages

48V Telecom and Base Station Power Supplies

Networking Equipment, Servers

Automotive and Industrial Control

PARAMETER LTC3703-5 LTC3703

Maximum V

60V 100V

MOSFET Gate Drive 4.5V to 15V 9.3V to 15V

3.7V 8.7V

–

3.1V 6.2V

LTC3703

MODE/SYNC

SET

COMP

MAX

INV

RUN/SS

GND

BOOST

DRV

BGRTN

68µF

1000pF

2200pF

10k

100Ω

113k

8.06k

10Ω

15k

30k

0.1µF

15V TO 100V

Si7456DP

8µH

MBR1100

OUT

12V

270µF

16V

BAS19

9.3V TO 15V

0.1µF

3703 F01

10µF

1µF

Si7456DP

22µF

25V

470pF

LOAD (A)

EFFICIENCY (%)

100

3703 F01b

= 25V

= 50V

= 75V

Figure 1. High Efficiency High Voltage Step-Down Converter

Efficiency vs Load Current

L, LT , LT C , LT M , Linear Technology and the Linear logo are registered trademarks and

ThinSOT and No R

SENSE

are trademarks of Linear Technology Corporation. All other trademarks

are the property of their respective owners. Protected by U.S. Patents, including 5408150,

5055767, 6677210, 5847554, 5481178, 6304066, 6580258.

Summary of content (34 pages)

PAGE 1
LTC3703 100V Synchronous Switching Regulator Controller Description Features n n n n n n n n n n n n n n n High Voltage Operation: Up to 100V Large 1Ω Gate Drivers No Current Sense Resistor Required Step-Up or Step-Down DC/DC Converter Dual N-Channel MOSFET Synchronous Drive Excellent Line and Load Transient Response Programmable Constant Frequency: 100kHz to 600kHz ±1% Reference Accuracy Synchronizable up to 600kHz Selectable Pulse-Skip Mode Operation Low Shutdown Current: 50µA Typ Programmable Current
PAGE 2
LTC3703 Absolute Maximum Ratings (Note 1) Supply Voltages VCC, DRVCC ........................................... –0.3V to 15V (DRVCC – BGRTN), (BOOST – SW)........ –0.3V to 15V BOOST.................................................. –0.3V to 115V BGRTN........................................................ –5V to 0V VIN Voltage............................................... –0.3V to 100V SW Voltage (Note 10).................................. –1V to 100V RUN/SS Voltage.......................................
PAGE 3
LTC3703 Electrical Characteristics The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VCC = DRVCC = VBOOST = VIN = 10V, VMODE/SYNC = VINV = VSW = BGRTN = 0V, RUN/SS = IMAX = open, RSET = 25k, unless otherwise specified.
PAGE 4
LTC3703 Electrical Characteristics Note 1: Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. Exposure to any Absolute Maximum Rating condition for extended periods may affect device reliability and lifetime. Note 2: The LTC3703E is guaranteed to meet performance specifications from 0°C to 85°C. Specifications over the –40°C to 85°C operating temperature range are assured by design, characterization and correlation with statistical process controls.
PAGE 5
LTC3703 Typical Performance Characteristics VCC Shutdown Current vs Temperature Reference Voltage vs Temperature 70 Normalized Frequency vs Temperature 0.803 1.20 1.15 55 50 45 40 0.802 NORMALIZED FREQUENCY 60 REFERENCE VOLTAGE (V) VCC CURRENT (µA) 65 0.801 0.800 0.799 30 –50 –25 0 25 50 75 100 125 150 TEMPERATURE (°C) 0.798 –50 –25 0 0.95 0.90 VCC = 10V PEAK SOURCE CURRENT (A) 1.4 2.6 Driver Peak Source Current vs Supply Voltage 3.0 1.6 VCC = 10V 1.2 2.4 RDS(ON) (Ω) 2.2 2.0 1.
PAGE 6
LTC3703 Typical Performance Characteristics RUN/SS Sink Current vs SW Voltage 6 25 5 20 4 3 2 1 0 Max % DC vs RUN/SS Voltage 100 IMAX = 0.3V 90 80 15 MAX DUTY CYCLE (%) RUN/SS SINK CURRENT (µA) RUN/SS PULLUP CURRENT (µA) RUN/SS Pull-Up Current vs VCC Voltage 10 5 0 6 8 10 12 VCC VOLTAGE (V) 14 16 0 0.1 0.2 0.3 0.4 0.5 |SW| VOLTAGE (V) 0.6 0.7 12 1.0 2.0 1.5 RUN VOLTAGE (V) 2.
PAGE 7
LTC3703 Pin Functions (GN16/G28) MODE/SYNC (Pin 1/Pin 6): Pulse-Skip Mode Enable/Sync Pin. This multifunction pin provides pulse-skip mode enable/disable control and an external clock input for synchronization of the internal oscillator. Pulling this pin below 0.8V or to an external logic-level synchronization signal disables pulse-skip mode operation and forces continuous operation. Pulling the pin above 0.8V enables pulse-skip mode operation.
PAGE 8
LTC3703 Functional Diagram RSET 2 fSET GN16 OVERCURRENT 12µA 4µA – 5 IMAX RMAX + 50mV – ± + RUN/SS 5 – CSS 0.9V 3.2V 1 ± CHIP SD + UVSD OTSD MODE/SYNC SYNC DETECT EXT SYNC OSC 4 R2 R1 16 12 DB – INV REVERSE CURRENT BOOST TG COMP 0.8V FB VCC + FORCED CONTINUOUS 3 INV + + FB – ÷ % DC LIMIT SW – PWM + DRIVE LOGIC VIN DRVCC BG +MIN– +MAX– BGRTN INV VCC (<15V) 0.76V 0.84V VIN 15 14 CB M1 13 11 M2 10 9 6 L1 OVER TEMP OT SD BANDGAP VCC UVLO 0.
PAGE 9
LTC3703 Operation (Refer to Functional Diagram) drop in the feedback voltage relative to the reference. The COMP voltage then rises, increasing the duty ratio until the output feedback voltage again matches the reference voltage. In normal operation, the top MOSFET is turned on when the RS latch is set by the on-chip oscillator and is turned off when the PWM comparator trips and resets the latch.
PAGE 10
LTC3703 Operation (Refer to Functional Diagram) side MOSFETs can occur. To prevent this from occurring, the bottom driver return is brought out as a separate pin (BGRTN) so that a negative supply can be used to reduce the effect of the Miller pull-up. For example, if a –2V supply is used on BGRTN, the switch node dV/dt could pull the gate up 2V before the VGS of the bottom MOSFET has more than 0V across it.
PAGE 11
LTC3703 (Refer to Functional Diagram) For maximum protection, the LTC3703 current limit consists of a steady-state limit circuit and an instantaneous limit circuit. The steady-state limit circuit is a gm amplifier that pulls a current from the RUN/SS pin proportional to the difference between the SW and IMAX voltages. This current begins to discharge the capacitor at RUN/ SS, reducing the duty cycle and controlling the output voltage until the current regulates at the limit.
PAGE 12
LTC3703 Operation (Refer to Functional Diagram) Buck or Boost Mode Operation The LTC3703 has the capability of operating both as a step-down (buck) and step-up (boost) controller. In boost mode, output voltages as high as 80V can be tightly regulated. With the INV pin grounded, the LTC3703 operates in buck mode with TG driving the main (topside) switch and BG driving the synchronous (bottom side) switch.
PAGE 13
LTC3703 Applications Information The oscillator can also be synchronized to an external clock applied to the MODE/SYNC pin with a frequency in the range of 100kHz to 600kHz (refer to the MODE/SYNC Pin section for more details). In this synchronized mode, pulse-skip mode operation is disabled. The clock high level must exceed 2V for at least 25ns. As shown in Figure 8, the top MOSFET turn-on will follow the rising edge of the external clock by a constant delay equal to one-tenth of the cycle period.
PAGE 14
LTC3703 Applications Information breakdown specification. Since many high voltage MOSFETs have higher threshold voltages (typically, VGS(MIN) ≥ 6V), the LTC3703 is designed to be used with a 9V to 15V gate drive supply (DRVCC pin). For maximum efficiency, on-resistance RDS(ON) and input capacitance should be minimized. Low RDS(ON) minimizes conduction losses and low input capacitance minimizes transition losses.
PAGE 15
LTC3703 Applications Information The term (1 + δ) is generally given for a MOSFET in the form of a normalized RDS(ON) vs temperature curve, and typically varies from 0.005/°C to 0.01/°C depending on the particular MOSFET used. Multiple MOSFETs can be used in parallel to lower RDS(ON) and meet the current and thermal requirements if desired. The LTC3703 contains large low impedance drivers capable of driving large gate capacitances without significantly slowing transition times.
PAGE 16
LTC3703 Applications Information Since ∆IL increases with input voltage, the output ripple is highest at maximum input voltage. ESR also has a significant effect on the load transient response. Fast load transitions at the output will appear as voltage across the ESR of COUT until the feedback loop in the LTC3703 can change the inductor current to match the new load current value.
PAGE 17
LTC3703 Applications Information + C10 1µF 16V 1µF CIN 12V + VIN C9 4.7µF 6.3V VIN LT1613 SHDN GND TG L1 BG FB R17 110k 1% VIN TG DRVCC SW L2 10µH LTC3703 12V SW + R17 1M 1% CIN LTC3703 VCC D2 ZHCS400 VIN VIN + L1 VOUT 10V TO 15V VCC SW COUT DRVCC BG VOUT <10V + COUT BGRTN BGRTN 3703 F10b 3703 F10a Figure 10a. VCC Generated from 10V < VOUT < 15V Figure 10b.
PAGE 18
LTC3703 Applications Information Bottom MOSFET Source Supply (BGRTN) The bottom gate driver, BG, switches from DRVCC to BGRTN where BGRTN can be a voltage between ground and –5V. Why not just keep it simple and always connect BGRTN to ground? In high voltage switching converters, the switch node dV/dt can be many volts/ns, which will pull up on the gate of the bottom MOSFET through its Miller capacitance.
PAGE 19
LTC3703 Applications Information approach its ESR and the rolloff due to the capacitor will stop, leaving 6dB/octave and 90° of phase shift (Figure 11). GAIN (dB) GAIN OUT RB –6dB/OCT 0 FREQ + VREF –90 –180 –270 –360 –90 –180 3703 F13 –270 Figure 13. Type 2 Schematic and Transfer Function –360 Figure 11. Transfer Function of Buck Modulator C1 PHASE (DEG) GAIN (dB) So far, the AC response of the loop is pretty well out of the user’s control.
PAGE 20
LTC3703 Applications Information Feedback Component Selection Selecting the R and C values for a typical Type 2 or Type 3 loop is a nontrivial task. The applications shown in this data sheet show typical values, optimized for the power components shown. They should give acceptable performance with similar power components, but can be way off if even one major power component is changed significantly.
PAGE 21
LTC3703 Applications Information Finally, choose a convenient resistor value for R1 (10k is usually a good value).
PAGE 22
LTC3703 Applications Information However, note that the power dissipation equations for the MOSFETs at maximum output current in a boost converter are:  I  PMAIN = DMAX  MAX   1–DMAX  2 (1+ δ )RDS(ON) + 1 2  IMAX  V  (RDR )(CMILLER ) • 2 OUT  1–DMAX   1  1 +   ( f)  VCC – VTH(IL) VTH(IL)   1  2 PSYNC = –  IMAX ) (1+ δ )RDS(ON) (   1–DMAX  Boost Converter: Output Capacitor Selection In boost mode, the output capacitor requirements are more demanding due to the fact that the curre
PAGE 23
LTC3703 Applications Information expected duty cycle (minimum VIN) in order to ensure that the current limit does not kick in at loads < IO(MAX): VPROG = IO(MAX) 1–DMAX RDS(ON)(1+ δ)  V  =  OUT  IO(MAX) •RDS(ON)(1+ δ)  VIN(MIN)  GAIN (dB) AV PHASE (DEG) GAIN –12dB/OCT 0 0 –90 PHASE Once VPROG is determined, RIMAX is chosen as follows: –180 RIMAX = VPROG/12µA Note that in a boost mode architecture, it is only possible to provide protection for “soft” shorts where VOUT > VIN.
PAGE 24
LTC3703 Applications Information charge up the soft-start capacitor CSS. When the voltage on RUN/SS reaches 0.9V, the LTC3703 begins operating at its minimum on-time. As the RUN/SS voltage increases from 1.4V to 3V, the duty cycle is allowed to increase from 0% to 100%. The duty cycle control minimizes input supply inrush current and eliminates output voltage overshoot at start-up and ensures current limit protection even with a hard short. The RUN/SS voltage is internally clamped at 4V.
PAGE 25
LTC3703 Applications Information MODE/SYNC Pin (External Synchronization) The internal LTC3703 oscillator can be synchronized to an external oscillator by applying and clocking the MODE/ SYNC pin with a signal above 2VP-P . The internal oscillator locks to the external clock after the second clock transition is received. When external synchronization is detected, LTC3703 will operate in forced continuous mode.
PAGE 26
LTC3703 Applications Information 1. VCC supply current. The VCC current is the DC supply current given in the Electrical Characteristics table which powers the internal control circuitry of the LTC3703. Total supply current is typically about 2.5mA and usually results in a small (<1%) loss which is proportional to VCC. 2. DRVCC current is MOSFET driver current. This current results from switching the gate capacitance of the power MOSFETs.
PAGE 27
LTC3703 Applications Information With most output capacitors, several devices paralleled to get the ESR down will have so much capacitance that this drop term is negligible. Ceramic capacitors are an exception; a small ceramic capacitor can have suitably low ESR with relatively small values of capacitance, making this second drop term more significant.
PAGE 28
LTC3703 Applications Information Next, choose the top and bottom MOSFET switch. Since the drain of each MOSFET will see the full supply voltage 72V (max) plus any ringing, choose a 100V MOSFET to provide a margin of safety. Si7456DP has a 100V BVDSS, RDS(ON) = 25mΩ (max), δ = 0.009/°C, CMILLER = (19nC – 10nC)/50V = 180pF, VGS(MILLER) = 4.7V, θJA = 20°C/W. The power dissipation can be estimated at maximum input voltage, assuming a junction temperature of 100°C (30°C above an ambient of 70°C): 12 (10)2 [1+ 0.
PAGE 29
LTC3703 Applications Information 7. For optimum load regulation and true remote sensing, the top of the output resistor divider should connect independently to the top of the output capacitor (Kelvin connection), staying away from any high dV/dt traces. Place the divider resistors near the LTC3703 in order to keep the high impedance FB node short. 5. Place the small-signal components away from high frequency switching nodes (BOOST, SW, TG, and BG).
PAGE 30
LTC3703 Typical Applications 36V-72V Input Voltage to 5V/10A Step-Down Converter with Pulse Skip Mode Enabled VCC 9.3V TO 15V + RC1 10k CC1 470pF CC2 1000pF R2 21.5k 1% RC2 100Ω CC3 2200pF R1 113k 1% 22µF 25V 1 16 MODE/SYNC VIN RSET 25k 2 15 fSET BOOST LTC3703 3 14 TG COMP RMAX 20k 4 5 6 CSS 0.1µF 7 8 FB SW IMAX VCC INV RUN/SS GND DRVCC BG BGRTN VIN 36V TO 72V DB BAS19 M1 Si7852DP CB 0.1µF 13 L1 4.
PAGE 31
LTC3703 Package Description Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings. GN Package 16-Lead Plastic (Narrow .150 Inch) GNSSOP Package (Reference LTC DWG 05-08-1641 Rev Inch) B) 16-Lead Plastic SSOP# (Narrow .150 (Reference LTC DWG # 05-08-1641 Rev B) .189 – .196* (4.801 – 4.978) .045 ±.005 16 15 14 13 12 11 10 9 .254 MIN .009 (0.229) REF .150 – .165 .229 – .244 (5.817 – 6.198) .0165 ±.0015 .150 – .157** (3.810 – 3.988) .
PAGE 32
LTC3703 Package Description Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings. G Package 28-Lead Plastic SSOP (5.3mm) (Reference LTC DWG # 05-08-1640) 9.90 – 10.50* (.390 – .413) 28 27 26 25 24 23 22 21 20 19 18 17 16 15 1.25 ±0.12 7.8 – 8.2 5.3 – 5.7 0.42 ±0.03 7.40 – 8.20 (.291 – .323) 0.65 BSC 1 2 3 4 5 6 7 8 9 10 11 12 13 14 RECOMMENDED SOLDER PAD LAYOUT 2.0 (.079) MAX 5.00 – 5.60** (.197 – .221) 0° – 8° 0.09 – 0.25 (.0035 – .010) 0.55 – 0.
PAGE 33
LTC3703 Revision History (Revision history begins at Rev C) REV DATE DESCRIPTION PAGE NUMBER C 05/12 Note 10 Added. 4 3703fc Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
PAGE 34
LTC3703 Typical Application 12V to 24V/5A Synchronous Boost Converter + 1 RSET 30.1k 10k CC1 100pF 0.1µF RMAX 15k R1 113k 1% R2 3.92k 1% 2 4 5 7 8 DB CMDSH-3 16 B240A 15 BOOST LTC3703 3 14 TG COMP 6 CSS 0.1F MODE/SYNC VIN 22µF 25V fSET FB SW IMAX VCC INV DRVCC RUN/SS GND BG BGRTN CB 0.1µF 13 M1 Si7892DP COUT + 220µF 35V ×3 L1 3.