LINEAR TECHNOLOGY LT1950 Technical data

LOAD CURRENT (A)

0.5

EFFICIENCY (%)

95

90

85

80

75

70

4.5

1950 TA01b

1.5

2.5

3.5

5.5

VIN = 48V

VIN = 36V

VIN = 72V

FEATURES

LT1950

Single Switch PWM

Controller with Auxiliary

Boost Converter

U

DESCRIPTIO

■

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 1.23V Reference

■

Accurate Shutdown Pin Threshold with
Programmable Hysteresis

■

60ns Current Sense Delay

■

2.5V Auxiliary Reference Output

■

Synchronizable to an External Clock up to 1.5 • f

■

Current Mode Control

■

Small 16-Pin SSOP Package

U

APPLICATIO S

■

Telecom Power Supplies

■

Automotive Power Supplies

■

Portable Electronic Equipment

■

Isolated and Nonisolated DC/DC Converters

OSC

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 appli­cations. 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 shut­down. The LT1950 is available in a small 16-Pin SSOP
package.

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

TYPICAL APPLICATIO

2.5V

0.1µF

249k

4.99k

U

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

SLOPE

V

REF

BOOST

R

OSC

LT1950

BLANK

SYNC

GND

FB

V

V

SHDN

GATE

I

SENSE

PGND

COMP

V

IN2

SEC

10V

BIAS

IN

1µF

4.7k

0.022µF

470k

18k

100k

V

IN

MBRB20200

PA0581

Si7450

0.015Ω

47µH

47µF

1950 TA01a

V
26V
5A

Efficiency vs Load Current

OUT

1950fa

1

LT1950

TOP VIEW

GN PACKAGE

16-LEAD NARROW PLASTIC SSOP

1

2

3

4

5

6

7

8

16

15

14

13

12

11

10

9

COMP

FB

R

OSC

SYNC

SLOPE

V

REF

SHDN

GND

V

SEC

V

IN

BOOST

PGND

GATE

V

IN2

I

SENSE

BLANK

ABSOLUTE AXI U RATI GS

(Note 1)

BOOST ....................................................... –0.3V to 35V

V

, V

IN

FB, SYNC, V

COMP, BLANK ..........................................–0.3V to 3.5V

SLOPE ...................................................... –0.3V to 2.5V

I

SENSE

R

OSC

V

REF

Operating Junction Temperature Range
LT1950EGN/LT1950IGN (Notes 2, 5) ... –40°C to 125°C

Storage Temperature Range ..................–65°C to 150°C

Lead Temperature (Soldering, 10 sec).................. 300°C

ELECTRICAL CHARACTERISTICS

temperature range, otherwise specifications are at TA = 25°C. COMP = open, FB = 1.4V, R
= 0.1µF, SHDN = VIN, BLANK = 0V, I
specified.

PARAMETER CONDITIONS MIN TYP MAX UNITS

PWM Controller

Operating Input Voltage I

Minimum Start-Up Voltage I

VIN Quiescent Current I

V

Shutdown Current SHDN = 0V 5 20 µA

IN

Shutdown Threshold 3V < V

Shutdown Pin Current SHDN = 70mV Above Threshold –7 –10 –13 µA

Shutdown Pin Current Hysteresis SHDN = 100mV Below Threshold 4 7 10 µA

V

Quiescent Current I(V

IN2

V

Shutdown Current SHDN = 0V, V

IN2

V

(External Output)

REF

Output Voltage I

Line Regulation I

Load Regulation 0µA < I

Oscillator

Frequency: f

Minimum Programmable f

Maximum Programmable f

SYNC Input Resistance 20 kΩ

SYNC Switching Threshold 1.5 2.2 V

SYNC Frequency/f

f

OSC

V

ROSC

2

Line Reg 3V < VIN < 25V 0.05 0.15 %/V

WW

W

, SHDN ......................................... –0.3V to 25V

IN2

........................................... –0.3V to 6V

SEC

U

......................................................... –0.3V to 1V

.................................................................... –50µA

.................................................................... –10mA

The ● denotes the specifications which apply over the full operating

= 0V, V

SENSE

OSC

OSC

OSC

OSC

= 15V, GATE = 1nF, BOOST = open, VIN = 15V, V

IN2

= 0µA ● 3.0 25 V

VREF

= 0µA ● 2.6 3.0 V

VREF

= 0µA, FB = 1V, I

VREF

< 25V ● 1.261 1.32 1.379 V

IN

) = 0µA, FB = 1V, I

REF

= 2.7V (Boost Diode from VIN = 3V) 500 700 µA

IN2

= 0µA ● 2.425 2.500 2.575 V

VREF

= 0µA, 3V < VIN < 25V 1 5 mV

VREF

< 2.5mA 1 5 mV

VREF

R

= 249k, FB = 1V ● 170 200 230 kHz

OSC

R

= 499k 85 100 115 kHz

OSC

R

= 90.9k 440 500 560 kHz

OSC

(R

= 249k, f

OSC

9.5V < V

IN2

=200kHz), FB = 1V (Note 7) 1.25 1.5

OSC

< 25V 0.05 0.25 %/V

UUW

PACKAGE/ORDER I FOR ATIO

ORDER PART

NUMBER

LT1950EGN
LT1950IGN

GN PART MARKING

1950E

T

= 125°C, θJA = 110°C/W,

JMAX

(PIN 8) = 30°C/W

θ

JC

Consult LTC Marketing for parts specified with wider operating temperature ranges.

= 249k, SYNC = 0V, SLOPE = open, V

OSC

= 0.2V 2.3 3.0 mA

SENSE

= 0.2V 1.7 2.5 mA

SENSE

= 0V, unless otherwise

SEC

1950I

1V

REF

1950fa

LT1950

ELECTRICAL CHARACTERISTICS

temperature range, otherwise specifications are at TA = 25°C. COMP = open, FB = 1.4V, R
= 0.1µF, SHDN = VIN, BLANK = 0V, I

SENSE

= 0V, V

IN2

The ● denotes the specifications which apply over the full operating

= 249k, SYNC = 0V, SLOPE = open, V

= 15V, GATE = 1nF, BOOST = open, VIN = 15V, V

OSC

= 0V, unless otherwise

SEC

REF

specified.

PARAMETER CONDITIONS MIN TYP MAX UNITS
Error Amplifier

FB Reference Voltage 3V < VIN < 25V, V

FB Input Bias Current FB = FB Reference Voltage –75 –200 nA

Open Loop Voltage Gain VOL + 0.2V < COMP < VOH – 0.2 65 85 dB

Unity Gain Bandwidth (Note 6) 3 MHz

COMP Source Current FB = 1V, COMP = 1.6V ● –0.3 –1.1 –1.8 mA

COMP Sink Current FB = 1.4V, COMP = 1.6V 8 13 mA

COMP High Level: V

OH

FB = 1V, I

COMP

COMP Active Threshold Start of GATE Switching (Duty Cycle > 0%) 1.0 V

COMP Low Level: V

OL

FB = 1.4V, I

Current Sense

I

Maximum Threshold Duty Cycle < 10%, COMP = V

SENSE

I

Input Bias Current COMP = 2.5V, I

SENSE

Default Blanking Time FB = 1V, COMP = 2V, I

Adjustable Blanking Time FB = 1V, COMP = 2V, I

BLANK = 75k to Ground

Blanking Override Voltage– BLANK = Open, COMP = 2.5V (Note 4) 15 25 40 mV

Maximum Threshold

I

SENSE

Turn-Off Delay to Gate COMP = 2V 60 ns

Slope Compensation (Note 4) I

Max Threshold (DC < 10%) – (DC = 80%) (Note 4)

SENSE

Default, R

SLOPE

2x Default, R
3x Default, R

Internal Switcher

Boost Switch I

LIMIT

Boost Switch Off Time V

V

: Boost Disable 3V < VIN < 10V ● 9.5 11.0 11.75 V

IN2

V

: Boost Disable Hysteresis 3V < VIN < 10V –1.0 V

IN2

V

: Gate Enable 3V < VIN < 10V, FB = 1V (Note 4) ● 7.0 8.2 9.27 V

IN2

V

: Gate Enable Hysteresis 3V < VIN < 10V, FB = 1V (Note 4) –0.6 V

IN2

V

= 8V, 3V < VIN < 10V 70 125 180 mA

IN2

= 8V, 3V < VIN < 10V 250 500 1000 ns

IN2

GATE Driver Output

GATE Rise Time FB = 1V, V

GATE Fall Time FB = 1V, V

GATE Clamp Voltage I

GATE Low Level I

GATE High Level I

GATE

GATE

I

GATE

GATE

I

GATE

Maximum Duty Cycle FB = 1V, f

IN2

IN2

= 0µA, COMP = 2.5V, FB = 6V 11.5 13 14.5 V

= 20mA 0.25 0.4 V
= 200mA 1.2 1.75 V

= –20mA, V
= –200mA, V

OSC

+ 0.2V < COMP < VOH – 0.2 ● 1.205 1.230 1.254 V

OL

= – 250µA 2.5 V

= 250µA 0.15 V

COMP

90 100 110 mV

SENSE

OH

= I

Max Threshold –125 –170 –250 µA

SENSE

= 75mV 110 ns

SENSE

= 75mV 290 ns

SENSE

= ∞ 14 mV

= 8k 28 mV

SLOPE

= 3.3k 42 mV

SLOPE

= 12V, CL = 1nF (Notes 3, 6) 50 ns

= 12V, CL = 1nF (Notes 3, 6) 30 ns

= 12V, COMP = 2.5V, FB = 6V 10 V

IN2

= 12V, COMP = 2.5V, FB = 6V 9.75 V

IN2

= 200kHz 90 95 97 %

1950fa

3

LT1950

ELECTRICAL CHARACTERISTICS

temperature range, otherwise specifications are at TA = 25°C. COMP = open, FB = 1.4V, R
= 0.1µF, SHDN = VIN, BLANK = 0V, I

SENSE

= 0V, V

IN2

The ● denotes the specifications which apply over the full operating

= 249k, SYNC = 0V, SLOPE = open, V

= 15V, GATE = 1nF, BOOST = open, VIN = 15V, V

OSC

= 0V, unless otherwise

SEC

REF

specified.

PARAMETER CONDITIONS MIN TYP MAX UNITS

Maximum Duty Cycle Clamp V

V

Input Bias Current 0V < V

SEC

Note 1: Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.

Note 2: The LT1950EGN is guaranteed to meet performance specifications
from 0°C to 125°C operating junction temperature. Specifications over the
–40°C to 125°C operating junction temperature range are assured by
design, characterization and correlation with statistical process controls.
The LT1950IGN is guaranteed over the full –40°C to 125°C operating
junction temperature range.

= 1.4V, FB = 1V, COMP = V

SEC

< 2.8V –0.3 –1.0 µA

SEC

OH

63 75 87 %

Note 4: Guaranteed by correlation to static test.
Note 5: This IC includes overtemperature protection that is intended to

protect the device during momentary overload conditions. Junction
temperature will exceed 125°C when overtemperature protection is active.
Continuous operation above the specified maximum operating junction
temperature may impair device reliability.

Note 6: Guaranteed but not tested.
Note 7: Maximum recommended SYNC frequency = 500kHz.

Note 3: Rise and Fall times are between 10% and 90% levels.

UW

TYPICAL PERFOR A CE CHARACTERISTICS

Switching Frequency vs

FB Voltage vs Temperature

1.26

1.25

1.24

1.23

FB VOLTAGE (V)

1.22

1.21

1.20
–50

–25 0

25 75

TEMPERATURE (°C)

50 100 125

1950 G01

Temperature

240

230

220

210

200

190

FREQUENCY (kHz)

180

170

160

–25 25 75

–50

0

TEMPERATURE (°C)

50

100

1950 • G02

125

VIN Shutdown IQ vs Temperature

16

SHDN = 0V

14

12

(µA)

Q

10

8

6

SHUTDOWN I

IN

V

4

2

0

–25 25 75

–50

0

TEMPERATURE (°C)

50

100

125

1950 • G03

Shutdown Threshold vs
Temperature

1.45

1.40

1.35

1.30

SHUTDOWN THRESHOLD (V)

1.25

1.20
–25 25 75

–50

0

TEMPERATURE (°C)

4

I

Maximum I

Threshold vs

SENSE

Temperature

125

120

115

110

105

THRESHOLD (mV)

100

95

SENSE

90

85

MAXIMUM I

80

50

100

125

1950 • G04

75

–50

0

–25

TEMPERATURE (°C)

50

25

75

100

125

1950 G05

270

250

230

210

190

170

CURRENT (µA)

150

SENSE

I

130

110

90

Pin Current vs

SENSE

Temperature

–50 0

–25

50

25

TEMPERATURE (°C)

100

125

1950 G06

1950fa

75

UW

TYPICAL PERFOR A CE CHARACTERISTICS

BLANK Override Threshold –
I

Maximum Threshold vs

SENSE

Temperature

40

35

30

25

20

MAXIMUM THRESHOLD (mV)

BLANK OVERRIDE THRESHOLD

15

SENSE

–I

10

–50

–25 0

25 75

TEMPERATURE (°C)

50 100 125

1950 G07

Minimum VIN Start-Up Voltage vs
Temperature (V

3.00

2.75

2.50

START-UP VOLTAGE (V)

IN

2.25

MINIMUM V

2.00
–25 25 75 125

–50

050

TEMPERATURE (°C)

Boosted)

IN2

100

1950 G08

3.1

2.9

2.7

2.5

(mA)

2.3

Q

I

IN

V

2.1

1.9

1.7

1.5

LT1950

V

vs Temperature

IN IQ

050

–25 25 75 125

–50

TEMPERATURE (°C)

100

1950 G09

SHDN Input Current *(–1) vs
Temperature

14

12

SHDN = SHDN THRESHOLD + 70mV

10

8

6

4

SHDN = SHDN THRESHOLD – 100mV

SHDN INPUT CURRENT*(–1) (µA)

2

0

–50 25 75

–25 0

TEMPERATURE (°C)

V

: GATE Enable

IN2

vs Temperature

9.2

8.7

GATE ENABLE

8.2

GATE ENABLE (V)

IN2

V

7.7

7.2
–50 25 75

–25 0

HYSTERESIS

GATE DISABLE

TEMPERATURE (°C)

50 100 125

1950 G10

50 100 125

1950 G13

SHDN Current Hysteresis vs
Temperature

11

10

9

8

7

6

5

SHDN CURRENT HYSTERESIS (µA)

4

3

–50

V

IN2

050

–25 25 75 125

TEMPERATURE (°C)

: BOOST Disable

vs Temperature

13.0

12.5

12.0

11.5

11.0

10.5

BOOST DISABLE (V)

IN2

V

10.0

9.5

9.0
–50

BOOST DISABLE

HYSTERESIS

BOOST RE-ENABLE

–25 25 75

0

TEMPERATURE (°C)

GATE Rise/Fall Time vs
GATE Capacitance

125

TA = 25°C

100

1950 G11

100

75

50

GATE RISE/FALL TIME (ns)

25

0

1000

0

BOOST Switch I

t

r

t

f

3000

2000

GATE CAPACITANCE (pF)

vs

LIMIT

4000

5000

1950 G12

Temperature

250

200

(mA)

LIMIT

150

100

BOOST SWITCH I

50

100

125

1950 G14

50

–50 25 75

–25 0

TEMPERATURE (°C)

50 100 125

1950 G15

1950fa

5

LT1950

UW

TYPICAL PERFOR A CE CHARACTERISTICS

BOOST Switch Off Time vs
Temperature

700

600

500

400

BOOST SWITCH OFF TIME (ns)

300

–50 25 75

–25 0

TEMPERATURE (°C)

U

50 100 125

1950 G16

UU

Maximum Duty Cycle vs

Voltage

V

SEC

100

90

80

70

60

50

MAXIMUM DUTY CYCLE (%)

40

30

0.8

MAX DUTY CYCLE = (105/V

1.25V < V
= 25°C

T

A

1.6 2.0 2.4 2.8 3.2

1.2
V

SEC

PI FU CTIO S

COMP (Pin 1): The COMP pin is the output of the error
amplifier. The error amplifier is a true op amp which allows
the use of an RC network to be connected between the
Comp and FB pins to compensate the feedback loop for
optimum transient response. The peak switch current in
the external MOSFET will be proportional to the voltage on
the COMP pin. Typical operating voltage range for this pin
is 1V to 2.5V.

FB (Pin 2): The FB pin is the inverting input to the error
amplifier. The output voltage is set with a resistor divider.
The error amplifier adjusts the peak switch current to
maintain the FB pin voltage at the value of the internal
reference voltage of 1.23V.

R

(Pin 3): A resistor from the R

OSC

programs the operating frequency of the LT1950. Operat­ing frequency range is 100kHz to 500kHz. Nominal voltage
on the R

pin is 1V.

OSC

pin to ground

OSC

GATE Clamp Voltage vs
Temperature

16

15

14

13

12

GATE CLAMP VOLTAGE (V)

11

10

–50

050

–25 25 75 125

TEMPERATURE (°C)

100

1950 G18

< 2.8V

SEC

VOLTAGE (V)

SEC

)%

1950 G17

SLOPE (Pin 5): The SLOPE pin is used to adjust the
amount of slope compensation. Leaving the pin open
circuit results in a default level of slope compensation. The
amount of slope compensation can be adjusted above this
default level by connecting a resistor from the SLOPE pin
to the V

V

REF

pin.

REF

(Pin 6): The V

pin is the output of an internal 2.5V

REF

reference. This pin is capable of sourcing up to 2.5mA for
external use. It is recommended that the V

REF

pin is

bypassed to ground with a 0.1µF ceramic capacitor.

SHDN (Pin 7): The SHDN pin is used to put the device into
a low power shutdown state. In shutdown the VIN supply
current drops to 5µA. The SHDN pin has an accurate
threshold of 1.32V which can be used to program an
undervoltage lockout threshold. Input current levels on
the SHDN pin can be used to program hysteresis into the
undervoltage lockout levels.

SYNC (Pin 4): The SYNC pin is used to synchronize the
internal oscillator to an external clock signal. The pin is
directly logic compatible and can be driven with any signal
with a duty cycle of 10% to 90%. If the SYNC function is
not used the pin can be left open circuit or connected to
ground.

6

GND (Pin 8): The GND pin is the analog ground for the
internal circuitry of the LT1950. Sensitive circuitry such as
the feedback divider, frequency setting resistor, reference
bypass capacitor should be tied directly to this pin. See the
Applications Information section for recommendations
on ground connections.

1950fa