8

FN9238.0

October 19, 2005

OUT - This is the drive output to the power switching device.

It is a high current output capable of driving the gate of a

power MOSFET with peak currents of 1.0A. This GATE

threshold.

supply current will depend on the load applied to OUT. Total

average output current. Knowing the operating frequency, f,

and the MOSFET gate charge, Qg, the average output

current can be calculated from:

possible.

VREF - The 5.00V reference voltage output. +1.0/-1.5%

tolerance over line, load and operating temperature. Bypass

to GND with a 0.1

µF to 3.3µF capacitor to filter this output as

needed.

Functional Description

Features

The ISL8843 current mode PWM makes an ideal choice for

low-cost flyback and forward topology applications. With its

greatly improved performance over industry standard parts,

it is the obvious choice for new designs or existing designs

which require updating.

Oscillator

The ISL8843 has a sawtooth oscillator with a programmable

frequency range to 2MHz, which can be programmed with a

resistor from VREF and a capacitor to GND on the RTCT

pin. (Please refer to Figure 4 for the resistor and capacitance

required for a given frequency.)

Soft-Start Operation

Soft-start must be implemented externally. One method,

illustrated below, clamps the voltage on COMP.

Gate Drive

The ISL8843 is capable of sourcing and sinking 1A peak

current. To limit the peak current through the IC, an optional

external resistor may be placed between the totem-pole

output of the IC (OUT pin) and the gate of the MOSFET. This

small series resistor also damps any oscillations caused by

the resonant tank of the parasitic inductances in the traces of

the board and the FET’s input capacitance.

Slope Compensation

For applications where the maximum duty cycle is less than

50%, slope compensation may be used to improve noise

immunity, particularly at lighter loads. The amount of slope

compensation required for noise immunity is determined

empirically, but is generally about 10% of the full scale

current feedback signal. For applications where the duty

cycle is greater than 50%, slope compensation is required to

prevent instability.

Slope compensation may be accomplished by summing an

external ramp with the current feedback signal or by

subtracting the external ramp from the voltage feedback

error signal. Adding the external ramp to the current

feedback signal is the more popular method.

From the small signal current-mode model [1] it can be

shown that the naturally-sampled modulator gain, Fm,

without slope compensation, is

where Sn is the slope of the sawtooth signal and Tsw is the

duration of the half-cycle. When an external ramp is added,

the modulator gain becomes

where Se is slope of the external ramp and

The criteria for determining the correct amount of external

ramp can be determined by appropriately setting the

damping factor of the double-pole located at the switching

frequency. The double-pole will be critically damped if the

Q-factor is set to 1, over-damped for Q < 1, and under-

damped for Q > 1. An under-damped condition may result in

current loop instability.

where D is the percent of on time during a switching cycle.

Setting Q = 1 and solving for Se yields

Qg

f

×

=

(EQ. 5)

FIGURE 5. SOFT-START

VREF

COMP

GND

Fm

1

SnTsw

--------------------

=

(EQ. 6)

Fm

1

Sn Se

+

()Tsw

---------------------------------------

1

----------------------------

==

(EQ. 7)

1 Se

Sn

-------

+

=

(EQ. 8)

Q

1

π m

–

() 0.5

–

()

-------------------------------------------------

=

(EQ. 9)

1

π

--- 0.5

+



 1

1D

–

------------- 1

–





=

(EQ. 10)

ISL8843