6

LT1249

amplifier.

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.

GTDR (Pin 8): The MOSFET gate driver is a 1.5A fast totem

pole output. It is clamped at 15V. Capacitive loads like

MOSFET gates may cause overshoot. A gate series resis-

tor of at least 5

Ω will prevent the overshoot.

Error Amplifier

The error amplifier has a 100dB DC gain and 1.5MHz unity-

gain frequency. It is internally clamped at 12V. The nonin-

verting input is tied to the 7.5V reference.

Current Amplifier

signal to the current loop that is controlled by the current

sense resistor. The current amplifier is a transconductance

load. The inverting input is internally tied to GND. The

noninverting input is tied to the multiplier output. The

output is internally clamped at 8V. Output resistance is

about 4M; DC loading should be avoided because it will

lower the gain and introduce offset voltage at the inputs

which becomes a false reference signal to the current loop

and can distort line current. Note that in the current

averaging operation, high gain at twice the line frequency

is necessary to minimize line current distortion. Because

condition, 11mV will be present at the inputs of the current

amplifier if gain is rolled off to 450 at 120Hz (1nF in series

less than 100mV, lower gain will distort the current loop

reference signal and line current. If signal gain at the

100kHz switching frequency is too high, the system

behaves 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

100kHz and more than 300 at 120Hz.

Multiplier

The multiplier is a current multiplier with high noise

immunity in a high power switching environment. The

current gain is:

With a square function, because of the lower gain at light

power load, system stability is maintained and line current

distortion caused by the AC ripple fed back to the error

amplifier is minimized. Note that switching ripple on the

high impedance lines could get into the multiplier from the

internal 25k resistor in series with the low impedance

multiplier current input so that only a capacitor from the

multiplier output current is limited to 250

µA. Figure 1

shows the multiplier transfer curves.

0

300

150

0

1249 G04

250

500