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LT3798IMSE-PBF

Isolated No Opto-Coupler Flyback Controller with Active PFC

Linear Technology 

LT3798

TYPICAL PERFORMANCE CHARACTERISTICS TA = 25°C, unless otherwise noted.

Power Factor vs Input Voltage

1.00

0.99

0.98

0.97

0.96

0.95

0.94

0.93

0.92

0.91 PAGE 17 SCHEMATIC:

UNIVERSAL

0.90

90 110 130 150 170 190 210 230 250 270

VIN (VAC)

3798 G15

Efficiency vs Input Voltage

100

PAGE 17 SCHEMATIC:

UNIVERSAL

90

80

70

60

90 110 130 150 170 190 210 230 250 270

VIN (VAC)

3798 G16

PIN FUNCTIONS

CTRL1, CTRL2, CTRL3 (Pin 1, Pin 2, Pin 3): Current

Output Adjustment Pins. These pins control the output

current. The lowest value out of the three CTRL inputs is

compared to negative input of the operational amplifier.

VREF (Pin 4): Voltage Reference Output Pin. Typically 2V.

This pin drives a resistor divider for the CTRL pin, either

for analog dimming or for temperature limit/compensation

of output load. Can supply up to 200µA.

OVP (Pin 5): Overvoltage Protection. This pin accepts a

DC voltage to compare to the sample and hold’s voltage

output information. When output voltage information is

above the OVP, the part divides the minimum switching

frequency by 8, around 500Hz. This protects devices con-

nected to the output. This also allows the part to operate

with very little power consumption with no load to meet

energy star requirements.

VC (Pin 6): Compensation Pin for Internal Error Amplifier.

Connect a series RC from this pin to ground to compensate

the switching regulator. A 100pF capacitor in parallel helps

eliminate noise.

COMP+, COMP– (Pin 7, Pin 8): Compensation Pins for

Internal Error Amplifier. Connect a capacitor between these

two pins to compensate the internal feedback loop.

6

FB (Pin 9): Voltage Loop Feedback Pin. FB is used to

regulate the output voltage by sampling the third wind-

ing. If the converter is used in current mode, the FB pin

will normally be at a voltage level lower than 1.25V, and

will reach the steady state of 1.25V if it detects an open

output condition.

DCM (Pin 10): Discontinuous Conduction Mode Detection

Pin. Connect a capacitor and resistor in series with this

pin to the third winding.

VIN (Pin 11): Input Voltage. This pin supplies current to

the internal start-up circuitry and to the INTVCC LDO. This

pin must be locally bypassed with a capacitor. A 42V shunt

regulator is internally connected to this pin.

EN/UVLO (Pin 12): Enable/Undervoltage Lockout. A resis-

tor divider connected to VIN is tied to this pin to program

the minimum input voltage at which the LT3798 will turn

on. When below 1.25V, the part will draw 60µA with most

of the internal circuitry disabled and a 10µA hysteresis

current will be pulled out of the EN/UVLO pin. When above

1.25V, the part will be enabled and begin to switch and the

10µA hysteresis current is turned off.

INTVCC (Pin 13): Regulated Supply for Internal Loads

and GATE Driver. Supplied from VIN and regulates to 10V

(typical). INTVCC must be bypassed with a 4.7µF capacitor

placed close to the pin.

3798fa

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