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LTC1922-1

TYPICAL APPLICATIO

DESCRIPTIO

FEATURES

Synchronous Phase

Modulated Full-Bridge Controller

LOAD CURRENT (A)

EFFICIENCY (%)

1922 • TA01b

100

= 36V

= 48V

Efficiency

The LTC

1922-1 phase shift PWM controller provides all

of the control and protection functions necessary to imple-

ment a high performance, zero voltage switched, phase

shift, full-bridge power converter with synchronous recti-

fication. The part is ideal for developing isolated, low

voltage, high current outputs from a high voltage input

source. The LTC1922-1 combines the benefits of the full-

bridge topology with fixed frequency, zero voltage switch-

ing operation (ZVS). Adaptive ZVS circuity controls the

turn-on signals for each MOSFET independent of internal

and external component tolerances for optimal perfor-

mance.

The LTC1922-1 also provides secondary side synchro-

nous rectifier control. The device uses peak current mode

control with programmable slope comp and leading edge

blanking.

The LTC1922-1 features extremely low operating and

start-up currents to simplify off-line start-up and bias

circuitry. The LTC1922-1 also includes a full range of

protection features and is available in 20-pin through hole

(N) and surface mount (G) packages.

■

Telecommunications, Infrastructure Power Systems

■

Distributed Power Architectures

■

Server Power Supplies

■

High Density Power Modules

■

Adaptive DirectSense

Zero Voltage Switching

■

Integrated Synchronous Rectification Control for

Highest Efficiency

■

Output Power Levels from 50W to Kilowatts

■

Very Low Start-Up and Quiescent Currents

■

Compatible with Voltage Mode and Current Mode

Topologies

■

Programmable Slope Compensation

■

Undervoltage Lockout Circuitry with 4.2V Hysteresis

and Integrated 10.3V Shunt Regulator

■

Fixed Frequency Operation to 1MHz

■

50mA Outputs for Bridge Drive and Secondary Side

Synchronous Rectifiers

■

Soft-Start, Cycle-by-Cycle Current Limiting and

Hiccup Mode Short-Circuit Protection

■

5V, 15mA Low Dropout Regulator

■

20-Pin PDIP and SSOP Packages

, LTC and LT are registered trademarks of Linear Technology Corporation.

DirectSense is a trademark of Linear Technology Corporation.

LTC1922-1

1922 TA01a

ISOLATED

FEEDBACK

48V

OUT

3.3V

BIAS

SUPPLY

APPLICATIO S

Summary of content (24 pages)

PAGE 1
LTC1922-1 Synchronous Phase Modulated Full-Bridge Controller DESCRIPTIO U FEATURES ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ Adaptive DirectSenseTM Zero Voltage Switching Integrated Synchronous Rectification Control for Highest Efficiency Output Power Levels from 50W to Kilowatts Very Low Start-Up and Quiescent Currents Compatible with Voltage Mode and Current Mode Topologies Programmable Slope Compensation Undervoltage Lockout Circuitry with 4.2V Hysteresis and Integrated 10.
PAGE 2
LTC1922-1 U W W W ABSOLUTE AXI U RATI GS U W U PACKAGE/ORDER I FOR ATIO (Note 1) ORDER PART NUMBER TOP VIEW VCC to GND Low Impedance Source ......................... –0.3V to 10V (Chip Self Regulates at 10.3V) All Other Pins to GND (Low Impedance Source) ..................... –0.3V to 5.5V VCC (Current Fed) .................................................. 25mA VREF Output Current ................................ Self Regulated Outputs (A, B, C, D, E, F) Current .....................
PAGE 3
LTC1922-1 ELECTRICAL CHARACTERISTICS The ● denotes the specifications which apply over the full operating temperature range, otherwise specifications are at VCC = 9.5V, CT = 180pF, TA = TMIN to TMAX unless other wise noted. SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS OSCT Total Variation VCC = 6.5V to 9.5V 236 277 319 kHz OSCV CT RAMP Amplitude OSYT SYNC Threshold Measured on CT 3.6 3.85 4.2 V Measured on SYNC 1.6 1.8 2.
PAGE 4
LTC1922-1 U W TYPICAL PERFOR A CE CHARACTERISTICS Start-Up ICC vs VCC TA = 25°C TA = 25°C CT = 180pF 270 VCC (V) 100 FREQUENCY (kHz) 10.25 150 ICC (µA) 280 10.50 200 10.00 9.50 0 2 6 4 8 10 10 0 VCC (V) 30 20 ISHUNT (mA) 40 240 – 60 – 40 – 20 0 20 40 60 TEMPERATURE (°C) 50 Leading Edge Blanking Time vs RLEB VREF vs IREF 5.05 TA = 25°C 300 TJ = 25°C 5.00 TJ = 85°C VREF (V) BLANK TIME (ns) 250 200 150 4.95 4.90 TJ = –40°C 100 4.85 50 4.
PAGE 5
LTC1922-1 U W TYPICAL PERFOR A CE CHARACTERISTICS Delay Hysteresis Current vs Temperature Start-Up ICC vs Temperature 1.278 150 1.276 140 120 110 100 90 80 120 1.274 5 35 65 TEMPERATURE (°C) 95 125 CT = 3.0V 110 1.272 1.270 1.268 1.266 1.264 1.262 100 90 80 70 CT = 1.5V 60 1.260 50 40 1.256 –55 –25 5 35 65 TEMPERATURE (°C) 95 125 –55 –25 5 35 65 TEMPERATURE (°C) 1922 • G11 1922 • G10 95 125 1922 • G12 Delay Pin Threshold vs Temperature VCC Shunt Voltage vs Temperature 10.
PAGE 6
LTC1922-1 U U U PIN FUNCTIONS SYNC (Pin 1): Synchronization Input/Output for the Oscillator. Terminate SYNC with a 5.1k resistor to GND. RAMP (Pin 2): Input to Phase Modulator Comparator. The voltage on RAMP is internally level shifted by 400mV. CS (Pin 3): Input to Current Limit Comparators, Output of Slope Compensation Circuitry. COMP (Pin 4): Error Amplifier Output, Input to Phase Modulator. RLEB (Pin 5): Timing Resistor for Leading Edge Blanking.
PAGE 7
LTC1922-1 W BLOCK DIAGRA VCC VREF CT SYNC SBUS 15 11 20 1 9 5V UVLO SHUNT REG 10.25V “ON” 6V “0FF” REF AND LDO 1.2V ERROR AMPLIFIER FB 6 OSC 8 PDLY – 1.2V + 50k COMP 4 PHASE MODULATOR Q + T + PASSIVE DELAY 17 OUTB QB 0.4V RAMP 2 18 OUTA – 14.
PAGE 8
LTC1922-1 U OPERATIO Phase Shift Full-Bridge PWM 5) Optimized current mode control architecture. Conventional full-bridge switching power supply topologies are often employed for high power, isolated DC/DC and off-line converters. Although they require two additional switching elements, substantially greater power and higher efficiency can be attained for a given transformer size compared to the more common single-ended forward and flyback converters.
PAGE 9
LTC1922-1 U OPERATIO Each full cycle of the transformer has two distinct periods in which power is delivered to the output, and two “freewheeling” periods. The two sides of the external bridge have fundamentally different operating characteristics that become important when designing for ZVS over a wide load current range. The left bridge leg is referred to as the “passive” leg, while the right leg is referred to as the “active” leg.
PAGE 10
LTC1922-1 U OPERATIO .and must be considered when specifying the power transformer. If ZVS is required over the entire range of loads, a small commutating inductor is added in series with the primary to aid with the passive leg transition, since the leakage inductance alone is usually not sufficient and predictable enough to guarantee ZVS over the full load range. State 1. State 4 (Power Pulse 2) During power pulse 2, current builds up in the primary winding in the opposite direction as power pulse 1.
PAGE 11
LTC1922-1 U OPERATIO Zero Voltage Switching (ZVS) Adaptive Mode A lossless switching transition requires that the respective full-bridge MOSFETs be switched to the “ON” state at the exact instant their drain to source voltage is zero. Delaying the turn-on results in lower efficiency due to circulating current flowing in the body diode of the primary side MOSFET rather than its low resistance channel. Premature turn-on produces hard switching of the MOSFETs, increasing noise and power dissipation.
PAGE 12
LTC1922-1 U OPERATIO PDLY corresponding to several volts across the MOSFET, the LTC1922-1 can “anticipate” a zero voltage VDS and signal the external driver and switch to turn-on. The amount of anticipation can be tailored for any application by modifying the upper divider resistor(s). The LTC1922-1 DirectSense circuitry sources a trimmed current out of PDLY and ADLY after a low to high level transition occurs.
PAGE 13
LTC1922-1 U OPERATIO Off-Line Bias Supply Generation If a regulated bias supply is not available to provide VCC voltage to the LTC1922-1 and supporting circuitry, one must be generated. Since the power requirement is small, approximately 1W, and the regulation is not critical, a simple open-loop method is usually the easiest and lowest cost approach. One method that works well is to add a winding to the main power transformer, and post regulate the resultant square wave with an L-C filter (see Figure␣ 5a).
PAGE 14
LTC1922-1 U OPERATIO 2. The LTC1922-1 can either synchronize other PWMs, or be synchronized to an external frequency source or PWM chip. See Figure 6 for details. CT OF SLAVE(S) IS 1.25 CT OF MASTER. LTC1922-1 CT 5.1k MASTER CT SYNC CT 5.1k SYNC CT LTC1922-1 1k 1k SYNC • • 5.1k • UP TO 5 SLAVES LTC1922-1 CT CT (33µA/V(CT)). Thus, at the peak of CT, this current is approximately 125µA and is output from the CS pin.
PAGE 15
LTC1922-1 U OPERATIO PULSE BY PULSE CURRENT LIMIT φMOD + Q S Q S – R OVERLOAD CURRENT LIMIT + S Q – UVLO ENABLE 4.1V R 12µA SS 0.4V + 600mV PWM LOGIC – RCS 400mV Q H = SHUTDOWN OUTPUTS UVLO ENABLE + CS PWM LATCH CSS – Q 1922 F08 Figure 8. Current Sense/Fault Circuitry Detail The pulse-by-pulse comparator has a 400mV nominal threshold, which can reduce sense resistor losses by 67% compared to previous solutions. This corresponds to 3W in a 200W, 48V to 3.3V converter.
PAGE 16
LTC1922-1 U OPERATIO POWER LOSS (W) 2.0 RS = 0.025 1.8 VIN = 48V VO = 3.3V 1.6 LO = 2.2µH 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0 0 5 10 15 20 25 30 OUTPUT CURRENT (A) 35 40 1922 • F09 Figure 9. RSENSE Power Loss vs IOUT If RAMP and CS are connected together: RCS = 0.4V – (125µA • RSLOPE) IP (PEAK) IP (PEAK) = IO(MAX) + VIN(MAX) • 2 • DMIN LMAG • f CLK 2 • N • EFF VO (1 – DMIN ) LOUT • f CLK• N + where: N = Transformer turns ratio If RAMP and CS are separated RCS 0.
PAGE 17
LTC1922-1 U OPERATIO Phase Modulator (MA-MD or MB-MC) conduct and cause current in an output inductor to increase. This current is seen on the primary of the power transformer divided by the turns ratio. Since the current sense resistor is connected between GND and the two bottom bridge transistors, a voltage proportional to the output inductor current will be seen across RSENSE. The high side of RSENSE is also connected to RAMP and CS, usually through a small resistor (RSLOPE).
PAGE 18
LTC1922-1 U OPERATIO Selecting the Power Stage Components Perhaps the most critical part of the overall design of the converter is selecting the power MOSFETs, transformer, inductors and filter capacitors. Tremendous gains in efficiency, transient performance and overall operation can be obtained as long as a few simple guidelines are followed with the phase shifted full-bridge topology. Power Transformer This guide is aimed at selecting readily available standard “off the shelf” transformers.
PAGE 19
LTC1922-1 U OPERATIO where: COSS = MOSFET D-S capacitance lMAG = magnetizing inductance fSW = switching frequency D = duty cycle LL = leakage inductance For a 48V to 3.3V/5V, 200W converter, the following values were derived: fSW LMAG LCOM LOUT : 300kHz : 100µH : 0.9µH : 2.2µH Turns Ratio (N) = 2.5 Output Capacitors Output capacitor selection has a dramatic impact on ripple voltage, dynamic response to transients and stability.
PAGE 20
LTC1922-1 U OPERATIO snubber capacitor and/or a very low resistance turn-off driver. If synchronous rectifier MOSFETs are used on the secondary, the same general guidelines apply. Keep in mind, however, that the BVDSS rating needed for these can be greater than VIN(MAX)/N, depending on how well the secondary is snubbed.
PAGE 21
LTC1922-1 U OPERATIO slower. A linearized SPICE macromodel of the control loop is very helpful tool to quickly evaluate the frequency response of various compensation networks. Polymer Electrolytic (see Figure 12) 1/(2πCCRI) sets a low frequency pole. 1/(2πCCRF) sets the low frequency zero. The zero frequency should coincide with the worstcase lowest output pole frequency.
PAGE 22
LTC1922-1 U OPERATIO the start of a new power cycle. A noninverting MOSFET driver such as the LTC1693-1 (Figure 14) is used so that a single signal transformer with secondary center tap can be employed to translate the drive signals from the primary to the secondary side. In the event of overcurrent shutdown, or UVLO condition, both synchronous rectification MOSFETs are driven on in order to protect the load circuitry.
PAGE 23
LTC1922-1 U PACKAGE DESCRIPTIO Dimensions in inches (millimeters) unless otherwise noted. G Package 20-Lead Plastic SSOP (0.209) (LTC DWG # 05-08-1640) 7.07 – 7.33* (0.278 – 0.289) 20 19 18 17 16 15 14 13 12 11 7.65 – 7.90 (0.301 – 0.311) 1 2 3 4 5 6 7 8 9 10 5.20 – 5.38** (0.205 – 0.212) 1.73 – 1.99 (0.068 – 0.078) 0° – 8° 0.13 – 0.22 (0.005 – 0.009) 0.65 (0.0256) BSC 0.55 – 0.95 (0.022 – 0.037) 0.05 – 0.21 (0.002 – 0.008) 0.25 – 0.38 (0.010 – 0.
PAGE 24
LTC1922-1 U TYPICAL APPLICATIO Synchronous Phase Shifted Full Bridge 48V to 3.3V, 40A Isolated Converter 10V D2 BAS21 R3 470Ω D1 BAS21 C3 2.2µF VCC2 VCC1 1 R6 470Ω +VOUT VLOW Q3 R4 10Ω Q4 FMMT718 8T L4 2.2µH P C8, 1nF 100V R2 10k 0.125W 2T VHIGH Q5 FMMT619 Q6 FMMT619 Q11 Q13 FMMT718 C14 0.1µF Q10 + + + C11 470µF 6V Q7 R9 0.06Ω 1W T3 TTWB-1010 10V 22µF 25V Q8 C9 470µF 6V Q12 Q14 FMMT718 R10 470Ω Q9 C4 + 470µF 6V C7 + 470µF 6V L3 2.