Datasheet

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LT1950

1950fa

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Wide Input Range: 3V to 25V

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Programmable Volt-Second Clamp

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Output Power Levels from 25W to 500W

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Auxiliary Boost Converter Provides 10V Gate Drive

from V

as Low as 3V

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Programmable Operating Frequency (100kHz to

500kHz) with One External Resistor

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Programmable Slope Compensation

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Programmable Leading Edge Blanking

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±2% Internal 1.23V Reference

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Accurate Shutdown Pin Threshold with

Programmable Hysteresis

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60ns Current Sense Delay

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2.5V Auxiliary Reference Output

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Synchronizable to an External Clock up to 1.5 • f

OSC

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Current Mode Control

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Small 16-Pin SSOP Package

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Telecom Power Supplies

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Automotive Power Supplies

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Portable Electronic Equipment

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Isolated and Nonisolated DC/DC Converters

Single Switch PWM

Controller with Auxiliary

Boost Converter

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

SLOPE

REF

BOOST

OSC

BLANK

SYNC

GND

IN2

SEC

SHDN

GATE

SENSE

PGND

COMP

LT1950

1950 TA01a

0.1µF

249k

4.99k

4.7k

0.022µF

1µF

470k

18k

10V

BIAS

100k

MBRB20200

47µH

47µF

OUT

26V

0.015Ω

Si7450

2.5V

PA0581

Efficiency vs Load Current

LOAD CURRENT (A)

0.5

EFFICIENCY (%)

4.5

1950 TA01b

1.5

2.5

3.5

5.5

= 48V

= 36V

= 72V

36V to 72V DC to 26V/5A (Single Switch) Forward Converter

The LT

1950 is a wide input range, forward, boost, flyback

and SEPIC controller that drives an N-channel power

MOSFET with few external components required.

A resistor programmable duty cycle clamp can be used to

generate a volt-second clamp for forward converter appli-

cations. An internal boost switcher is available for creating

a separate supply for the output gate driver, allowing 10V

gate drive from input voltages as low as 3V. The LT1950’s

operating frequency can be set with an external resistor

over a 100kHz to 500kHz range and a SYNC pin allows the

part to be synchronized to an external clock. Additional

programmability exists for leading edge blanking and

slope compensation.

A fast current sense comparator achieves 60ns current

sense delay and the error amplifier is a true voltage mode

error amplifier, allowing a wide range of compensation

networks. An accurate shutdown pin with programmable

hysteresis is available for undervoltage lockout and shut-

down. The LT1950 is available in a small 16-Pin SSOP

package.

APPLICATIO S

FEATURES

DESCRIPTIO

TYPICAL APPLICATIO

Summary of content (20 pages)

PAGE 1
LT1950 Single Switch PWM Controller with Auxiliary Boost Converter DESCRIPTIO U FEATURES ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ Wide Input Range: 3V to 25V Programmable Volt-Second Clamp Output Power Levels from 25W to 500W Auxiliary Boost Converter Provides 10V Gate Drive from VIN as Low as 3V Programmable Operating Frequency (100kHz to 500kHz) with One External Resistor Programmable Slope Compensation Programmable Leading Edge Blanking ±2% Internal 1.
PAGE 2
LT1950 U W W W ABSOLUTE AXI U RATI GS U W U PACKAGE/ORDER I FOR ATIO (Note 1) BOOST ....................................................... –0.3V to 35V VIN, VIN2, SHDN ......................................... –0.3V to 25V FB, SYNC, VSEC ........................................... –0.3V to 6V COMP, BLANK .......................................... –0.3V to 3.5V SLOPE ...................................................... –0.3V to 2.5V ISENSE .........................................................
PAGE 3
LT1950 ELECTRICAL CHARACTERISTICS The ● denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. COMP = open, FB = 1.4V, ROSC = 249k, SYNC = 0V, SLOPE = open, VREF = 0.1µF, SHDN = VIN, BLANK = 0V, ISENSE = 0V, VIN2 = 15V, GATE = 1nF, BOOST = open, VIN = 15V, VSEC = 0V, unless otherwise specified. PARAMETER CONDITIONS MIN TYP MAX UNITS 1.205 1.230 1.
PAGE 4
LT1950 ELECTRICAL CHARACTERISTICS The ● denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. COMP = open, FB = 1.4V, ROSC = 249k, SYNC = 0V, SLOPE = open, VREF = 0.1µF, SHDN = VIN, BLANK = 0V, ISENSE = 0V, VIN2 = 15V, GATE = 1nF, BOOST = open, VIN = 15V, VSEC = 0V, unless otherwise specified. PARAMETER CONDITIONS MIN Maximum Duty Cycle Clamp VSEC = 1.4V, FB = 1V, COMP = VOH VSEC Input Bias Current 0V < VSEC < 2.
PAGE 5
LT1950 U W TYPICAL PERFOR A CE CHARACTERISTICS BLANK Override Threshold – ISENSE Maximum Threshold vs Temperature Minimum VIN Start-Up Voltage vs Temperature (VIN2 Boosted) 3.1 3.00 35 30 25 20 15 10 –50 –25 50 25 75 0 TEMPERATURE (°C) 100 2.9 2.7 2.75 VIN IQ (mA) MINIMUM VIN START-UP VOLTAGE (V) BLANK OVERRIDE THRESHOLD –ISENSE MAXIMUM THRESHOLD (mV) 40 2.50 0 25 50 75 1.
PAGE 6
LT1950 U W TYPICAL PERFOR A CE CHARACTERISTICS BOOST Switch Off Time vs Temperature Maximum Duty Cycle vs VSEC Voltage 100 MAX DUTY CYCLE = (105/VSEC)% 1.25V < VSEC < 2.8V TA = 25°C MAXIMUM DUTY CYCLE (%) 90 600 500 400 80 70 60 50 50 25 75 0 TEMPERATURE (°C) 100 125 30 0.8 15 14 13 12 11 40 300 –50 –25 16 GATE CLAMP VOLTAGE (V) 700 BOOST SWITCH OFF TIME (ns) GATE Clamp Voltage vs Temperature 1.2 1.6 2.0 2.4 VSEC VOLTAGE (V) 1950 G16 2.8 3.
PAGE 7
LT1950 U U U PI FU CTIO S BLANK (Pin 9): The BLANK pin is used to adjust the leading edge blanking period of the current sense amplifier during FET turn-on. Shorting the BLANK pin to ground provides a default blanking period of approximately 110ns. A resistor from the BLANK pin to ground increases the blanking period up to 290ns for RBLANK = 75k. ISENSE (Pin 10): The ISENSE pin is the current sense input for the control loop.
PAGE 8
LT1950 W BLOCK DIAGRA VIN VREF VSEC BOOST VIN2 15 6 16 14 11 VIN2 VREF = INTERNAL + EXTERNAL SUPPLY + SWITCHING PREREGULATOR FIXED OFF TIME 11V (125mA CURRENT LIMIT) 2.5V (SOURCE 2.5mA EXTERNALLY) DISABLE – PGND (VIN) (2.6V) U/V LOCKOUT + (VIN2) (8V) U/V LOCKOUT 8V 1.23V – (105/VSEC)% – – + ENABLE MAX DC + CLAMP 1.32V 3µA – (TYPICAL 200kHz) SHDN 7 OSC SYNC 4 S ±1A DRIVER Q 12 GATE + (100-500)khz R SLOPE COMP RAMP 13 PGND ROSC 3 SLOPE 5 1.
PAGE 9
LT1950 U OPERATIO increases the range of MOSFETs that can be selected and allows applications requiring high gate drive with a large swing in VIN voltage. When VIN2 exceeds 8V, the GATE output driver is enabled. The GATE switches between 0V and VIN2 at a constant frequency set by a resistor from the ROSC pin to ground. When VIN2 reaches 11V, the internal switcher at the BOOST pin is disabled to save power and only re-enabled when VIN2 drops below 10V.
PAGE 10
LT1950 U W U U APPLICATIO S I FOR ATIO LT1950 Input Supplies, VREF Output and GATE Enable VIN = VIN2 Operation VIN is the main input supply for the LT1950. VIN2 is the input supply for the LT1950 output driver. VIN2 can be provided by shorting the VIN2 pin to the VIN pin or by generating VIN2 using the BOOST pin. Waveforms of VIN, VIN2, VREF and GATE switching are shown in Figures 2 and 3. Figure 2 represents low VIN operation with VIN2 generated using the B00ST pin.
PAGE 11
LT1950 U W U U APPLICATIO S I FOR ATIO Shutdown and Undervoltage Lockout Figure 4 shows how to program undervoltage lockout (UVLO) for the VIN supply. Typically, UVLO is used in situations where the input supply is current limited, or has a relatively high source resistance. A switching regulator draws constant power from the source, hence source current increases as source voltage drops.
PAGE 12
LT1950 U U W U APPLICATIO S I FOR ATIO N-channel power MOSFET even though VIN voltage is very low. High GATE drive voltage reduces MOSFET RDS(ON), improves efficiency and increases the range of MOSFETs that can be selected. A small switching regulator at the BOOST pin, with fixed current limit and fixed off time, generates the VIN2 supply.
PAGE 13
LT1950 U W U U APPLICATIO S I FOR ATIO The SYNC pin is used to synchronize the LT1950 main oscillator to an external clock. The SYNC pin can be driven directly from a logic level output, requiring less than 0.8V for a logic level low and greater than 2.2V for a logic level high. Duty cycle must be between 10% and 90%. When synchronizing the part, slope compensation will be reduced by approximately SYNC f/fOSC.
PAGE 14
LT1950 U U W U APPLICATIO S I FOR ATIO Programming Leading Edge Blank Time Programming Volt-Second Clamp For PWM controllers driving external MOSFETs, noise can be generated during GATE rise time due to various parasitic effects. This noise can disturb the input to the current sense comparator (ISENSE) and cause premature turn-off of the external MOSFET. The LT1950 provides programmable leading edge blanking of the current sense comparator to avoid this effect.
PAGE 15
LT1950 U W U U APPLICATIO S I FOR ATIO ability of the VSEC pin to follow input voltage and control maximum switch duty cycle. Forward Converter Applications The LT1950 provides sophisticated control of the simplest forward converter topology (single primary switch, see Q1 Figure 11). A significant problem in a single switch forward converter topology is transformer reset. Optimum transformer utilization requires maximum duty cycles.
PAGE 16
LT1950 U W U U APPLICATIO S I FOR ATIO +VIN 36V TO 72V INPUT R1 4.7k 1 2 R17, 210k 3 4 C1 0.1µF 5 6 7 R4, 18k 8 LT1950 COMP FB VSEC VIN ROSC BOOST SYNC PGND SLOPE GATE VREF VIN2 SHDN ISENSE GND BLANK R5 470k 16 R7 255Ω CU1 1µF UV 13 8 1 D1 BAS516 U2A LTC1693-1 7 D2 BAT760 6 3 U2B LTC1693-1 5 4 CG SYNC R13 270Ω 560Ω R3 4.7k D3 BAT760 0.1µF 6 1 5 2 4 3 2 3 T2 U4 HCPL-M453 10VBIAS C6 4.7µF 1µF X5R C9, 33nF R2 4.
PAGE 17
LT1950 U W U U APPLICATIO S I FOR ATIO High Efficiency, Isolated 26V 5A Output, Nonsynchronous Forward Converter 94 93 92 Figure 13 illustrates a nonsynchronous forward converter based on the LT1950 to provide a highly efficient, 26V 5A isolated output from 48V input. The LT1950-based converter using a single switch topology and utilizing the LT1950s adaptive maximum duty cycle clamp is a simple and highly optimized solution. Peak efficiencies of 92.8% (Figure 12) are achievable.
PAGE 18
LT1950 U W U U APPLICATIO S I FOR ATIO 3.3V BIAS +VOUT R1 10.5k C1 2200pF R3 18k 1 2 3 R4 133k R2 1.21k R6 35.7k LT1950 COMP VSEC FB ROSC 4 R5 SYNC 16.2k 5 SLOPE 6 VREF 7 C2 SHDN 0.1µF 8 GND VIN BOOST PGND GATE VIN2 ISENSE BLANK R7 71.5k VIN C3 4.7µF 16V 16 15 L1 4.7µH CIN 10µF 50V TDK L2* 14 13 D2 BAS516 12 R9, 47Ω 9 C6 0.01µF R8 47k C5 10µF D1 50V MBRD660CT Q1 Si7456 11 10 VIN 4V TO 36V L3* C4 4.7µF 16V R10 0.010Ω +VOUT 12V, 1.
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LT1950 U PACKAGE DESCRIPTIO GN Package 16-Lead Plastic SSOP (Narrow .150 Inch) (Reference LTC DWG # 05-08-1641) .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) .0250 TYP RECOMMENDED SOLDER PAD LAYOUT 1 .015 ± .004 × 45° (0.38 ± 0.10) .007 – .0098 (0.178 – 0.249) 2 3 4 5 6 7 .053 – .068 (1.351 – 1.727) 8 .004 – .0098 (0.102 – 0.249) 0° – 8° TYP .016 – .050 (0.
PAGE 20
LT1950 RELATED PARTS PART NUMBER DESCRIPTION COMMENTS LT1534 Ultralow Noise 2A Switching Regulator Reduces Conducted and Radiated EMI, Low Switching Harmonics, 20kHz to 250kHz Switching Frequency LT1619 Low Voltage Current Mode Controller 1.9V ≤ VIN ≤ 18V, 300kHz Operation, Boost, Flyback, SEPIC LT1681/LT3781 Dual Transistor Synchronous Forward Controller Operation Up to 72V Maximum LTC1693 High Speed MOSFET Driver 1.