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LT1952/LT1952-1

19521fe

TYPICAL APPLICATION

DESCRIPTION

Single Switch Synchronous

Forward Controller

The LT

1952/LT1952-1 are current mode PWM controllers

optimized to control the forward converter topology, using

one primary MOSFET. The LT1952/LT1952-1 provide

synchronous rectiﬁer control, resulting in extremely

high efﬁciency. A programmable Volt-Second clamp

provides a safeguard for transformer reset that prevents

saturation. This allows a single MOSFET on the primary

side to reliably run at greater than 50% duty cycle for high

MOSFET, transformer and rectiﬁer utilization. The devices

include soft-start for controlled exit from shutdown and

undervoltage lockout. A precision 107mV current limit

threshold, independent of duty cycle, combines with soft-

start to provide hiccup short-circuit protection. The LT1952

is optimized for micropower bootstrap start-up from high

input voltages. The LT1952-1 allows start-up from lower

input voltages. Programmable slope compensation and

leading edge blanking allow optimization of loop bandwidth

with a wide range of inductors and MOSFETs. Each device

can be programmed over a 100kHz to 500kHz frequency

range and the part can be synchronized to an external

clock. The error ampliﬁer is a true op amp, allowing a wide

range of compensation networks. The LT1952/LT1952-1

are available in a small 16-pin SSOP package.

36V to 72V Input, 12V at 20A Semi-Regulated Bus Converter

FEATURES

APPLICATIONS

Synchronous Rectiﬁer Control for High Efﬁciency

Programmable Volt-Second Clamp

Output Power Levels from 25W to 500W

Low Current Start-Up

(LT1952: 460µA; V

On/Off = 14.25V/8.75V)

(LT1952-1: 400µA; V

On/Off = 7.75V/6.5V)

True PWM Soft-Start

Low Stress Short-Circuit Protection

Precision 107mV Current Limit Threshold

Adjustable Delay for Synchronous Timing

Accurate Shutdown Threshold with Programmable

Hysteresis

Programmable Slope Compensation

Programmable Leading Edge Blanking

Programmable Frequency (100kHz to 500kHz)

 SynchronizabletoanExternalClockupto1.5•f

OSC

Internal 1.23V Reference

2.5V External Reference

Current Mode Control

Small 16-Pin SSOP Package

Telecommunications Power Supplies

Industrial and Distributed Power

Isolated and Non Isolated DC/DC Converters

PA0905

SUPPLY FROM BIAS

WINDING OF T1

Si7370

×2

PH4840

×2

PA1494.242

47µF

16V

X5R

×2

OUT

12V

20A

220pF

560Ω

1952 TA01

REF

COMP

SS_MAXDC

SD_V

SEC

SYNC

GND

OUT

SENSE

SOUT

LT1952/

LT1952-1

BLANK DELAY

OSC

0.1µF

40k

16V

340k

13k

52.3k

40k 40k

PGND

178k

100k

Si7450

0.005Ω

SYNC

LTC3900

10µF

0.1µF

12V Bus Converter

OUT

vs V

(V)

OUT

(V)

1952 TA01b

L, LT, LTC and LTM are registered trademarks and ThinSOT is a trademark of Linear

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

Summary of content (28 pages)

PAGE 1
LT1952/LT1952-1 Single Switch Synchronous Forward Controller DESCRIPTION FEATURES n n n n n n n n n n n n n n n n n Synchronous Rectifier Control for High Efficiency Programmable Volt-Second Clamp Output Power Levels from 25W to 500W Low Current Start-Up (LT1952: 460µA; VIN On/Off = 14.25V/8.75V) (LT1952-1: 400µA; VIN On/Off = 7.75V/6.
PAGE 2
LT1952/LT1952-1 ABSOLUTE MAXIMUM RATINGS PIN CONFIGURATION (Note 1) TOP VIEW VIN (Note 8)................................................ –0.3V to 25V SYNC, SS_MAXDC, SD_VSEC, ISENSE, OC.....– 0.3V to 6V COMP, BLANK, DELAY................................ –0.3V to 3.5V FB .................................................................– 0.3V to 3V ROSC...................................................................... –50µA VREF .....................................................................
PAGE 3
LT1952/LT1952-1 ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the specified operating junction temperature range, otherwise specifications are at TA = 25°C (Note 2). COMP = open, FB = 1.4V, ROSC = 178k, SYNC = 0V, SS_MAXDC = VREF, VREF = 0.1µF, SD_VSEC = 2V, BLANK = 121k, DELAY = 121k, ISENSE = 0V, OC = 0V, OUT = 1nF, VIN = 15V, SOUT = open, unless otherwise specified.
PAGE 4
LT1952/LT1952-1 ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the specified operating junction temperature range, otherwise specifications are at TA = 25°C (Note 2). COMP = open, FB = 1.4V, ROSC = 178k, SYNC = 0V, SS_MAXDC = VREF, VREF = 0.1µF, SD_VSEC = 2V, BLANK = 121k, DELAY = 121k, ISENSE = 0V, OC = 0V, OUT = 1nF, VIN = 15V, SOUT = open, unless otherwise specified. PARAMETER CONDITIONS COMP Low Level: VOL I(COMP) = 250µA MIN TYP MAX 0.15 0.
PAGE 5
LT1952/LT1952-1 ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the specified operating junction temperature range, otherwise specifications are at TA = 25°C (Note 2). COMP = open, FB = 1.4V, ROSC = 178k, SYNC = 0V, SS_MAXDC = VREF, VREF = 0.1µF, SD_VSEC = 2V, BLANK = 121k, DELAY = 121k, ISENSE = 0V, OC = 0V, OUT = 1nF, VIN = 15V, SOUT = open, unless otherwise specified.
PAGE 6
LT1952/LT1952-1 TYPICAL PERFORMANCE CHARACTERISTICS 1.25 500 245 1.23 1.22 1.21 1.20 –50 –25 50 25 0 75 TEMPERATURE (°C) 100 VIN = 15V 450 SD_VSEC = 0V 230 VIN SHUTDOWN CURRENT (µA) SWITCHING FREQUENCY (kHz) 1.24 FB VOLTAGE (V) VIN Shutdown Current vs Temperature Switching Frequency vs Temperature FB Voltage vs Temperature 215 200 185 170 –25 50 25 0 75 TEMPERATURE (°C) 100 1952 G01 5.0 4.5 LT1952-1 300 4.0 250 50 25 0 75 TEMPERATURE (°C) 100 3.
PAGE 7
LT1952/LT1952-1 TYPICAL PERFORMANCE CHARACTERISTICS 12.5 FB = 1V COMP = 1.6V COMP SINK CURRENT (mA) COMP SOURCE CURRENT (mA) • (–1) 12.5 COMP Sink Current vs Temperature 10.0 7.5 CURRENT OUT OF PIN 5.0 –50 –25 50 25 0 75 TEMPERATURE (°C) 100 10.0 7.5 –25 50 25 0 75 TEMPERATURE (°C) 1952 G10 OC THRESHOLD 80 40 0 0 0.5 1.0 2.0 1.
PAGE 8
LT1952/LT1952-1 TYPICAL PERFORMANCE CHARACTERISTICS 200 TA = 25°C TA = 25°C 150 160 RDELAY = 120k 600 tDELAY (ns) BLANK (ns) 800 240 tDELAY: SOUT Rise to OUT Rise vs RDELAY 400 tDELAY (ns) 1000 tDELAY: SOUT Rise to OUT Rise vs Temperature BLANK Duration vs RBLANK 100 80 50 200 RDELAY = 40k 0 0 20 0 –50 60 80 100 120 140 160 RBLANK (k) 40 –25 50 25 0 75 TEMPERATURE (°C) 100 1952 G26 90 OUT MAX DUTY CYCLE CLAMP (%) 75 tf 50 90 80 25 0 TA = 25°C SS_MAXDC = 2.5V SD_VSEC = 1.
PAGE 9
LT1952/LT1952-1 PIN FUNCTIONS COMP (Pin 1): Output Pin of the Error Amplifier. The error amplifier is an op amp, allowing various compensation networks to be connected between the COMP pin and FB pin for optimum transient response. The voltage on this pin corresponds to the peak current of the external FET. Full operating voltage range is between 0.8V and 2.5V corresponding to 0mV to 220mV at the ISENSE pin.
PAGE 10
LT1952/LT1952-1 TIMING DIAGRAM tDELAY: PROGRAMMABLE SYNCHRONOUS DELAY SOUT OUT SS_MAXDC FAULTS TRIGGERING SOFT-START VIN < 8.75V OR SD_VSEC < 1.32V (UVLO) OR OC > 107mV (OVERCURRENT) 0.8V (ACTIVE THRESHOLD) 0.45V (RESET THRESHOLD) 0.2V SOFT-START LATCH RESET: SOFT-START LATCH SET VIN > 14.25V (> 8.75V IF LATCH SET BY OC) AND SD_VSEC > 1.32V AND OC < 107mV AND SS_MAXDC < 0.45V 1952 F01 Figure 1. Timing Diagram BLOCK DIAGRAM VIN 15 VINON VINOFF – VREF SS_MAXDC 6 5 0.45V + 2.
PAGE 11
LT1952/LT1952-1 OPERATION Introduction The LT1952/LT1952-1 are current mode synchronous PWM controllers optimized for control of the simplest forward converter topology—using only one primary MOSFET. The LT1952/LT1952-1 are ideal for 25W to 500W power systems where very high efficiency and reliability, low complexity and cost are required in a small space. Key features of the LT1952/LT1952-1 include an adaptive maximum duty cycle clamp for the single primary MOSFET.
PAGE 12
LT1952/LT1952-1 OPERATION catastrophic damage. Many converters solve this problem by limiting the operational duty cycle of the MOSFET to 50% or less—or by using a fixed (non-adaptive) maximum duty cycle clamp with very large voltage rated MOSFETs. The LT1952/LT1952-1 provide a volt-second clamp to allow MOSFET duty cycles well above 50%. This gives greater power utilization for the MOSFET, rectifiers and transformer resulting in less space for a given power output.
PAGE 13
LT1952/LT1952-1 OPERATION Slope Compensation The current mode architecture requires slope compensation to be added to the current sensing loop to prevent subharmonic oscillations which can occur for duty cycles above 50%. Unlike most current mode converters which have a slope compensation ramp that is fixed internally, placing a constraint on inductor value and operating frequency, the LT1952/LT1952-1 have externally adjustable slope compensation.
PAGE 14
LT1952/LT1952-1 APPLICATIONS INFORMATION current (460µA LT1952; 400µA LT1952-1) allows a large resistor to be connected between system input supply and VIN . Once the part is turned on, input current increases to drive the IC (4.5mA) and the output drivers (IDRIVE). A large enough capacitor is chosen at the VIN pin to prevent VIN falling below its turn off threshold before an auxiliary winding in the converter takes over supply to VIN .
PAGE 15
LT1952/LT1952-1 APPLICATIONS INFORMATION Blanking is provided in 2 phases (Figure 6): The first phase automatically blanks during gate rise time. Gate rise times can vary depending on MOSFET type. For this reason the LT1952/LT1952-1 perform true ‘leading edge blanking’ by automatically blanking OC and ISENSE comparator outputs until OUT rises to within 0.5V of VIN or reaches its clamp level of 13V. The second phase of blanking starts after the leading edge of OUT has been completed.
PAGE 16
LT1952/LT1952-1 APPLICATIONS INFORMATION Programming Synchronous Rectifier Timing: SOUT to OUT delay (‘tDELAY’) The LT1952/LT1952-1 have an additional output SOUT which provides a ±50mA peak drive clamped to 12V. In applications requiring synchronous rectification for high efficiency, the LT1952/LT1952-1 SOUT provides a sync signal for secondary side control of the synchronous rectifier MOSFETs (Figure 11).
PAGE 17
LT1952/LT1952-1 APPLICATIONS INFORMATION Example calculation for (2) For RT = 35.7k, RB = 100k, VREF = 2.5V, RDELAY = 40k, fOSC = 200kHz and SD_VSEC = 1.32V, this gives SS_MAXDC(DC) = 1.84V, tDELAY = 40ns and k = 1 Maximum Duty Cycle Clamp = 1 • 0.522(1.84/1.32) – (40ns • 200kHz) = 0.728 – 0.008 = 0.
PAGE 18
LT1952/LT1952-1 APPLICATIONS INFORMATION (A) VIN > 14.25* (7.75V LT1952-1), and (B) SD_VSEC > 1.32V, and (C) OC < 107mV, and (D) SS_MAXDC < 0.45V (SS_MAXDC reset threshold) *VIN > 8.75V (6.5V LT1952-1) is ok for latch reset if the latch was only set by overcurrent condition in (3) above. SS_MAXDC Discharge Timing It can be seen in Figure 10 that two types of discharge can occur for the SS_MAXDC pin. In timing (A) the fault that caused the soft-start event has been removed before SS_MAXDC falls to 0.45V.
PAGE 19
LT1952/LT1952-1 APPLICATIONS INFORMATION VSS(t) = SS_MAXDC(DC) (1 – e(–t/RC)) to give: t = RC • (–1) • ln(1 – VSS/SS_MAXDC(DC)) where: VSS = SS_MAXDC voltage at time t SS_MAXDC(DC) = programmed DC voltage setting maximum duty cycle clamp = VREF(RB /(RT + RB) R = RCHARGE (Figure 11) = RT • RB /(RT + RB) C = CSS (Figure 11) Example (1) No Switching Period The period of no switching for the converter, when a soft-start event has occurred, depends on how far SS_MAXDC can fall before recharging occurs and how lo
PAGE 20
LT1952/LT1952-1 APPLICATIONS INFORMATION For SD_VSEC = 1.32V, fOSC = 200kHz and RDELAY = 40k This gives k = 1 and tDELAY = 40ns. Re-arranging the above equation to solve for SS_MAXDC = VSS(REG) = [0.6 + (tDELAY • fOSC)(SD_VSEC)]/(k • 0.522) = [0.6 + (40ns • 200kHz)(1.32V)]/(1 • 0.522) = (0.608)(1.32)/0.522 = 1.537V t(1.803) = 2.63e–4 • 1e–7 • (–1) • ln(1 – 1.803/1.84) = 2.63e–3 • (–1) • ln(0.02) = 1.03e–2 s Hence the time for SS_MAXDC to charge from its minimum reset threshold of 0.
PAGE 21
LT1952/LT1952-1 APPLICATIONS INFORMATION 98 EFFICIENCY (%) 96 94 IOUT (5A/DIV) 92 0A 90 VOUT (200mV/DIV) 88 86 VIN = 48V VOUT = 5V fOSC = 300kHz 5 0 15 10 LOAD CURRENT (A) 20 Figure 13. Output Voltage Transient Response (6A to 12A Load Step at 6A/µs) 1952 F12 Figure 12. LT1952-Based Synchronous Forward Converter Efficiency vs Load Current (For Circuit in Figure 14) +VIN 36V TO 72V SOUT 16 5 100k 22k 6 2 0.1µF 0.1µF 3 115k 4 15 VIN BLANK DELAY 1 COMP 9 12 40k 33k 1k 18.
PAGE 22
LT1952/LT1952-1 APPLICATIONS INFORMATION 48V to Isolated 12V, 20A (No Opto-Coupler) ‘Bus Converter’ achieves a high 94% at 20A (Figure 15). The solution is only slightly larger than 1/4 “brick” size and uses only ceramic capacitors for high reliability. The wide programmable range and accuracy of the LT1952/LT1952-1 Volt-Second clamp makes the LT1952/ LT1952‑1 an ideal choice for ‘Bus Converter’ applications where the Volt-Second clamp provides line regulation for the converter output.
PAGE 23
LT1952/LT1952-1 APPLICATIONS INFORMATION An LT1952-based synchronous forward converter provides the ideal solution for power supplies requiring high efficiency at low output voltages and high load currents. The 3.3V 40A solution in Figure 18 achieves peak efficiencies of 92.5% (Figure 17) by minimizing power loss due to rectification at the output.
PAGE 24
LT1952/LT1952-1 APPLICATIONS INFORMATION Bus Converter: Optimum Output Voltage Tolerance The Bus Converter applications shown on page 1 and in Figure 16, provide semi-regulated isolated outputs without the need for an optocoupler, optocoupler driver, reference or feedback network. The LT1952/LT1952-1Volt-Second clamp adjusts switch duty cycle inversely proportional to input voltage to provide an output voltage that is regulated against input line variations.
PAGE 25
LT1952/LT1952-1 TYPICAL APPLICATIONS Wide 18V to 72V Input, High Efficiency, 12V at 12A Output, Active Reset Forward Converter Fits in One-Eighth Brick Footprint +VIN +VIN 18V TO 72V +VB VUL L2 R1 Q10 1.5mH 2.2Ω PBSS8110 +VIN R5 10Ω VUL 6 VCC BOOST LTC4440-5 5 2 TG GND 3 INP R3 442Ω TS C25 10nF Q1 HAT2173 4 RCSL 0.006Ω L53 10µH OUT D1 BAS516 1 2 Q12 BC856T R15 115k 3 4 C13 0.47µF 5 6 C10 0.1µF R22 13.3k R28 309k R32 187k +VB C33 0.1µF 7 8 COMP FB ROSC SOUT VIN OUT 16 T1 G45R2-0405.
PAGE 26
LT1952/LT1952-1 TYPICAL APPLICATIONS High Efficiency 36V to 72V Input to 3.3V at 30A Output, Active Reset Forward Converter Fits in One-Eighth Brick Footprint +VIN +VB VUL L2 R1 Q10 1.5mH 2.2Ω PBSS8110 +VIN CIN 2.2µF ×2 –VIN R11 82k C34 0.22µF D2 BAS516 9, 10, 11 +VIN 2, 3 • 1 2 Q12 BC856T R15 115k 3 COMP FB VIN D11 BAS516 C10 0.1µF 5 6 7 R22 13.
PAGE 27
LT1952/LT1952-1 (Revision history begins at Rev E) REV DATE DESCRIPTION PAGE NUMBER E 5/11 MP-grade parts added, changes reflected throughout the data sheet 1-28 19521fe 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 28
LT1952/LT1952-1 PACKAGE DESCRIPTION GN Package 16-Lead Plastic SSOP (Narrow .150 Inch) (Reference LTC DWG # 05-08-1641 Rev B) .189 – .196* (4.801 – 4.978) .045 ±.005 .009 (0.229) REF 16 15 14 13 12 11 10 9 .254 MIN .150 – .165 .229 – .244 (5.817 – 6.198) .0165 ±.0015 .150 – .157** (3.810 – 3.988) .0250 BSC RECOMMENDED SOLDER PAD LAYOUT 1 .015 ± .004 × 45° (0.38 ± 0.10) .007 – .0098 (0.178 – 0.249) .0532 – .0688 (1.35 – 1.75) 2 3 4 5 6 7 8 .004 – .0098 (0.102 – 0.249) 0° – 8° TYP .016 – .