LT1248CS datasheet(7/12 Pages) LINER

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LT1248

PI FU CTIO S

Pin 12 (RSET): A resistor from RSET to GND sets the

oscillator charging current and the maximum multiplier

output current which is used to limit the maximum line

current.

IM(MAX) = 3.75V/RSET

Pin 13 (SS): Soft-Start. When either VCC or EN/SYNC goes

low, the SS pin will stay at 0V. With a capacitor from the

pin to GND, the 12

µAchargingcurrentslowlybringsupthe

SS to 8V; below 7.5V SS is the reference input to the

voltage amplifier. At supply dropout or EN/SYNC low, the

soft start capacitor will be quickly discharged.

Pin 14 (CSET): The capacitor from this pin to GND, and

RSET, determine oscillator frequency. The oscillator ramp

is 5V, and the frequency = 1.5/(RSET • CSET).

Pin 15 (VCC): This is the supply for the chip. The LT1248

has a very fast gate driver required to fast charge high

power MOSFET gate capacitance. High current spikes

occur during charging. For good supply bypass, a 0.1

µF

ceramic capacitor in parallel with a low ESR electrolytic

capacitor, 56

µF or higher is required in close proximity to

IC GND.

Pin 16 (GTDR): The MOSFET gate driver is a 1.5A fast

totem pole output. It is clamped at 15V, but capacitive

loads like MOSFET gates may cause overshoot. A gate

series resistor of at least 5

Ω will prevent the overshoot.

IAC (µA)

300

150

1248 G04

250

500

VAOUT = 7V

VAOUT = 6.5V

VAOUT = 6V

VAOUT = 5.5V

VAOUT = 5V

VAOUT = 4.5V

VAOUT = 4V

VAOUT = 3.5V

VAOUT = 3V

VAOUT = 2.5V

Figure 1. Multiplier Current IM vs IAC and VAOUT

APPLICATI

I FOR ATIO

Multiplier

The multiplier is a current multiplier with high noise

immunity in a high power switching environment. The

current gain is: IM=(IAC• IEA2)/(200µA)2,with IEA=(VAOUT

– 2V)/25k. With a square function, because of the lower

gain at light power load, system stability is maintained and

line current distortion caused by the line frequency AC

Error Amplifier

The error amplifier has a 100dB DC gain and 3MHz unity-

gain frequency. The output is internally clamped at 13.5V.

The noninverting input is tied to the 7.5V VREF through a

diode and can be pulled down from the SS (soft-start) pin.

Current Amplifier

The current amplifier has a 110dB DC gain, 3MHz unity-

gain frequency, and a 2V/

µs slew rate. It is internally

clamped at 8.5V. Note that in the current averaging opera-

tion, high gain at twice the line frequency is necessary to

minimize line current distortion. Because CAOUT may need

to swing 5V over one line cycle at high line condition,

14mV AC will be needed at the inputs of the current

amplifier for a gain of 350 at 120Hz. Especially at light load

when the current loop reference signal is small, lower gain

will distort the reference signal and line current. If signal

gain at switching frequency is too high, the system be-

haves more like a current mode system and can cause

subharmonic oscillation. Therefore, the current amplifier

should be compensated to have a gain of less than 15 at

the switching frequency, but more than 250 at twice the

line frequency.

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