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LTC3900

3900fb

Typical applicaTion

DescripTion

Synchronous Rectiﬁer Driver

for Forward Converters

FeaTures

applicaTions

48V Input Isolated DC/DC Converters

Isolated Telecom Power Supplies

High Voltage Distributed Power

Step-Down Converters

Industrial Control System Power Supplies

Automotive and Heavy Equipment

N-Channel Synchronous Rectiﬁer MOSFET Driver

Programmable Timeout Protection

Reverse Inductor Current Protection

Pulse Transformer Synchronization

Wide V

Supply Range: 4.5V to 11V

15ns Rise/Fall Times at V

= 5V, C

= 4700pF

Undervoltage Lockout

Small SO-8 Package

The LTC

3900 is a secondary-side synchronous recti-

ﬁer driver designed to be used in isolated forward con-

verter power supplies. The chip drives N-channel rectiﬁer

MOSFETs and accepts pulse sychronization from the

primary-side controller via a pulse transformer.

The LTC3900 incorporates a full range of protection for the

external MOSFETs. A programmable timeout function is

included that disables both drivers when the synchroniza-

tion signal is missing or incorrect. Additionally, the chip

senses the output inductor current through the drain-source

resistance of the catch MOSFET, shutting off the MOSFET

if the inductor current reverses. The LTC3900 also shuts

off the drivers if the supply voltage is too low.

ISOLATION

BARRIER

CS1

CS2

CS3

–

SYNC

TIMER

LTC3900

TMR

SYNC

TMR

VCC

OUT

3.3V

40A

36V TO 72V

3900 F01

OUT

REG

GND

270Ω

OPTO

COMP

GND

OUT

SENSE

OUT

COMP

GND

LT1952

470Ω

10mΩ

GATE

LT4430

OCI

LOAD CURRENT (A)

EFFICIENCY (%)

10 20

3900 F10b

5 15

OUT

= 3.3V

= 36V

= 72V

= 48V

Efﬁciency

Figure 1. Simpliﬁed Isolated Synchronous Forward Converter

L, LT, LTC, LTM, Linear Technology, Burst Mode and the Linear logo are registered trademarks

of Linear Technology Corporation. All other trademarks are the property of their respective

owners.

Summary of content (20 pages)

PAGE 1
LTC3900 Synchronous Rectifier Driver for Forward Converters Features n n n n n n n n Description N-Channel Synchronous Rectifier MOSFET Driver Programmable Timeout Protection Reverse Inductor Current Protection Pulse Transformer Synchronization Wide VCC Supply Range: 4.5V to 11V 15ns Rise/Fall Times at VCC = 5V, CL = 4700pF Undervoltage Lockout Small SO-8 Package The LTC®3900 is a secondary-side synchronous rectifier driver designed to be used in isolated forward converter power supplies.
PAGE 2
LTC3900 Absolute Maximum Ratings Pin Configuration (Note 1) Supply Voltage VCC.........................................................................12V Input Voltage CS –, TIMER............................... – 0.3V to (VCC +0.3V) SYNC....................................................... –12V to 12V Input Current CS+.....................................................................15mA Operating Junction Temperature Range (Note 2) LTC3900E............................................
PAGE 3
LTC3900 Electrical Characteristics The l denotes the specifications which apply over the specified operating junction temperature range, otherwise specifications are at TA = 25°C. VCC = 5V unless otherwise specified. (Notes 2, 3) SYMBOL PARAMETER CONDITIONS tTMRDIS Timer Discharge Time CTMR = 1000pF, RTMR = 4.7k MIN V TMRMAX Timer Pin Clamp Voltage CTMR = 1000pF, RTMR = 4.
PAGE 4
LTC3900 Typical Performance Characteristics Timeout vs Temperature Timeout vs VCC 5.25 TA = 25°C RTMR = 51k CTMR = 470pF 5.20 5.15 5.15 TIMEOUT (µs) 4.95 7 5.05 5.00 4.95 6 5 4 4.90 4.90 3 4.85 4.85 2 4.80 4.80 1 4.75 4.75 –75 –50 –25 5 8 7 VCC (V) 6 9 10 11 3900 G01 VCS(MAX) Clamp Voltage vs CS+ Input Current 18 VCC = 5V, 11V VCS(MAX) CLAMP VOLTAGE (V) 13 11 9 7 5 25 16 15 14 13 12 10 50 75 100 125 150 TEMPERATURE (°C) 0 CS SYNC Negative Threshold vs Temperature –1.
PAGE 5
LTC3900 Typical Performance Characteristics Propagation Delay vs CLOAD TA = 25°C VCC = 5V 110 80 70 SYNC TO FG 60 25 20 15 RISE TIME 10 SYNC TO CG 4 5 6 7 CLOAD (nF) 8 9 0 10 5 4 6 7 8 9 10 UNDERVOLTAGE LOCKOUT THRESHOLD VOLTAGE (V) 40 35 30 25 RISE TIME 15 FALL TIME 10 5 0 0 1 2 18 3 4 5 6 CLOAD (nF) 7 8 9 4.2 0 –75 –50 –25 11 3.8 3.6 FALLING EDGE 3.4 3.
PAGE 6
LTC3900 Pin Functions CS+, CS– (Pin 1, 2): Current Sense Differential Input. Connect CS+ through a series resistor to the drain of the external catch MOSFET, Q4. Connect CS– to the source. The LTC3900 monitors the CS inputs 250ns after CG goes high. If the inductor current reverses and flows into the MOSFET causing CS+ to rise above CS– by more than 10.5mV, the LTC3900 pulls CG low. See the Current Sense section for more details on choosing the resistance value for RCS1 to RCS3.
PAGE 7
LTC3900 Applications Information Overview In a typical forward converter topology, a power transformer is used to provide the functions of input/output isolation and voltage step-down to achieve the required low output voltage. Schottky diodes are often used on the secondary-side to provide rectification. Schottky diodes, though easy to use, result in a loss of efficiency due to relatively high voltage drops.
PAGE 8
LTC3900 Applications Information The timer circuit and current sense comparator in LTC3900 are used to prevent reverse current buildup in the output inductor. Timer Figure 3 shows the LTC3900 timer internal and external circuits. The timer operates by using an external R-C charging network to program the time-out period. On every negative transition at the SYNC input, the chip generates a 200ns pulse to reset the timer cap.
PAGE 9
LTC3900 Applications Information time. The current sinking capability of the circuit is around 1mA. The timeout function can be disabled by connecting the timer pin to GND. Current Sense The differential input current sense comparator is used for sensing the voltage across the drain-to-source terminals of Q4 through the CS+ and CS– pins. If the inductor current reverses into the Q4 causing CS+ to rise above CS– by more than 10.5mV, the LTC3900 pulls CG low.
PAGE 10
LTC3900 Applications Information The value of the resistors, RCS1, RCS2 and RCS3, should be calculated using the following formulas to meet both the threshold and clamp voltage requirements: k = 48 • IRIPPLE • RDS(ON) –1 RCS2 = {200 • VIN(MAX) • (NS/NP) –2200 • (1 + k)} /k RCS1 = k • RCS2 To minimize this delay and error, do not use resistance value higher than required and make the PCB trace from the resistors to the LTC3900 CS+/CS– pins as short as possible.
PAGE 11
LTC3900 Applications Information An alternative method of generating the SYNC pulse is shown in Figure 8. This circuit produces square SYNC pulses with amplitude dependent on the logic supply voltage. The SYNC pulse width can be adjusted with R1 and C1 without affecting the pulse amplitude. For nonisolated applications, the SYNC input can be driven directly by a bipolar square pulse. To reduce the propagation delay, make the positive and negative magnitude of the square wave much greater than the ±1.
PAGE 12
LTC3900 Applications Information the driver dissipation, the total gate charge QG is used. This parameter is found on the MOSFET manufacturers data sheet. 2. Connect the two MOSFET drain terminals directly to the transformer. The two MOSFET sources should be as close together as possible. The power dissipated in each LTC3900 MOSFET driver is: 3. Keep the timer, SYNC and VCC regulator circuit away from the high current path of Q3, Q4 and T1. PDRIVER = QG • VCC • fSW 4.
PAGE 13
LTC3900 typical applications 36V to 72V Input to 12V and 24V (or ± 12V), 2A Output Converter in 1/8th Brick Footprint VU1 • BCX56 1.5mH 2 • 2.2µF PDZ10B BAS516 • 33k 22k BC857BF 1 2 115k 1.2k LT1952-1 3 4 5 0.1µF 6 7 13.3k 8 COMP FB = 1.23V SOUT VIN ROSC OUT SYNC PGND MAXDC VR = 2.5V SD DELAY OC ISENSE BLANK GND 0.47µF 16 15 100pF FG VOUT1 12V 2A Q5 CG 33µF 16V 0.
PAGE 14
LTC3900 typical applications 18V to 40V Input to 14V at 14A Output Converter in 1/4 Brick Footprint PZTA42 VIN 18V TO 40V VU1 22k 1.5mH 6.8µF ×4 1 PDZ10B BAS516 VIN 40R2-4421.003 1 2 220pF 3 BAS516 4 5 255R BL OUT DRVCC RBL SGND VIN FB IN PGND EN 10 332k 9 • FG 8 VU1 7 255R 6 33k 1 2.2µF 57.6k 115k 3 4 0.47µF 5 0.1µF 6 7 13.3k 8 22k 165k 150µF PXE FB = 1.23V SOUT VIN ROSC OUT SYNC PGND MAXDC VR = 2.5V DELAY OC SD ISENSE GND BLANK 158k 16 0.
PAGE 15
LTC3900 typical applications 36V to 72V Input to 12V, 14A Output Converter in 1/8th Brick Footprint VU1 PZTA42 T1 PA0423 1• • 7 L2 1.5mH PDZ10B BAS516 82k VIN 36V TO 72V 10 6 2.2µF 2 L1 3µH 47k HAT2244 • 560R 5 5 370k 7 133k 3 82k 9 5 22k 0.1µF 12.4k SD/VSEC ROSC BLANK SS OUT OC ISENSE VIN GND 6 20k 1 47nF 2 97.6k 6 VR PGND COMP DELAY FB SOUT 14 470pF 7 1nF 11 15 8 3 + 1 SYNC CS GND CS– TIMER VCC 2 10k 4 1µF BCX55 4.7µF 13 12 • 0.010 VU1 CG 38.
PAGE 16
LTC3900 typical applications 18V to 72V Input to 12V at 13A Active Reset Converter Fits in 1/8th Brick Size PZTA42 VIN 18V TO 72V 33k VU1 VR2 2.2µF ×3 40R2-4444.004 1• • 7 1.5mH 10V BAS516 2 VIN 1, 6 3 2, 4 5 HAT2173 ×2 LTC4440 220pF 33k 2 BC857 174k 3 4 1µF 0.1µF 39.2K 5 6 7 13.3k 8 189k VR2 332k CG 0.22µF COMP FB = 1.23V ROSC VIN OUT SYNC PGND MAXDC DELAY VR = 2.5V SD OC ISENSE GND BLANK BAS521 680µH Si2325 BCX55 15 7.5V 10k BAS516 16 14 0.
PAGE 17
LTC3900 typical applications Synchronous Forward Converter With Pulse Skip Mode PZTA42 VIN 36V TO 72V VU1 82k 2.2µF 1 PDZ10B BAS516 PA1671 7 HA2165 • FG 10nF T1 PA0369 1.5nF 100µF 2.2R 10k B0540W R_DCM 33k 22k BC857 2 115k 3 4 0.47µF 0.1µF 5 6 7 13.3k 0.22µF LT1952-1 1 8 22.1k COMP FB = 1.23V SOUT VIN ROSC OUT SYNC PGND MAXDC DELAY VR = 2.5V SD OC ISENSE BLANK GND 158k B0540W 16 15 10k 2 3 CG 4 CS+ SYNC CS– TIMER CG GND VCC 1µF FG 38.
PAGE 18
LTC3900 Package Description S8 Package 8-Lead Plastic Small Outline (Narrow .150 Inch) (Reference LTC DWG # 05-08-1610) .050 BSC .189 – .197 (4.801 – 5.004) NOTE 3 .045 ±.005 8 .245 MIN 7 6 5 .160 ±.005 .150 – .157 (3.810 – 3.988) NOTE 3 .228 – .244 (5.791 – 6.197) .030 ±.005 TYP RECOMMENDED SOLDER PAD LAYOUT 1 .010 – .020 × 45° (0.254 – 0.508) NOTE: 1. DIMENSIONS IN INCHES (MILLIMETERS) 2. DRAWING NOT TO SCALE 3. THESE DIMENSIONS DO NOT INCLUDE MOLD FLASH OR PROTRUSIONS.
PAGE 19
LTC3900 Revision History (Revision history begins at Rev B) REV DATE DESCRIPTION B 5/11 Added H- and MP-grade parts. Reflected throughout the data sheet. PAGE NUMBER 1 to 20 3900fb 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 20
LTC3900 Typical Application 36V to 72V Input to 12V at 20A “No Optocoupler” Synchronous “Bus Converter” 47k VIN 36V TO 72V VU1 L1 2.4µH PA0815.002 • • 82k BCX55 BAS516 0.1µF 18V Si7370 ×2 12V 2.2µF, 100V ×2 7 115k 3 27k 0.47µF 0.1µF 9 5 59k 10k 6 1 2 SD_VSEC OUT ROSC VIN BLANK GND SS_MAXDC LT1952 PGND DELAY VR = 2.5V COMP FB = 1.