ISL6840, ISL6841, ISL6842, ISL6843, ISL6844, ISL6845

FN9124 Rev 13.00

Page 9 of 14

February 18, 2015

Pin Descriptions

RTCT - This is the oscillator timing control pin. The operational

frequency and maximum duty cycle are set by connecting a

resistor, RT, between VREF and this pin and a timing capacitor,

CT, from this pin to GND. The oscillator produces a sawtooth

waveform with a programmable frequency range up to

switching frequency, f, and the maximum duty cycle, Dmax,

can be calculated from Equations 1, 2, 3 and 4:

Figure 4 may be used as a guideline in selecting the capacitor

and resistor values required for a given frequency.

COMP - COMP is the output of the error amplifier and the input

of the PWM comparator. The control loop frequency

compensation network is connected between the COMP and

FB pins.

FB - The output voltage feedback is connected to the inverting

input of the error amplifier through this pin. The non-inverting

input of the error amplifier is internally tied to a reference

voltage.

CS - This is the current sense input to the PWM comparator.

The range of the input signal is nominally 0V to 1.0V and has

an internal offset of 100mV.

GND - GND is the power and small signal reference ground for

all functions.

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.

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

output current. Knowing the operating frequency, f, and the

MOSFET gate charge, Qg, the average output current can be

calculated in Equation 5:

ceramic capacitor as close to the VDD and GND pins as

possible.

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

tolerance over line, load and operating temperature. Bypass to

GND with a 0.1µF to 3.3µF capacitor to filter this output as

needed.

Functional Description

Features

The ISL684x current mode PWMs make 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 ISL684x family of controllers have 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 in Figure 5, clamps the voltage on COMP.

Gate Drive

The ISL684x family are 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. The minimum amount of slope compensation

required corresponds to 1/2 the inductor downslope. Adding

excessive slope compensation, however, results in a control

loop that behaves more as a voltage mode controller than as a

current mode controller.

Slope compensation may be added to the CS signal shown in

Figure 7.

tC 0.583 RT CT







(EQ. 1)

tD RT

–

CT

0.0083 RT 4.3

–



0.0083 RT 2.4

–



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





ln







(EQ. 2)

f

1tC tD

+





=

(EQ. 3)



=

(EQ. 4)

(EQ. 5)

Qg f



=

FIGURE 5. SOFT-START

VREF

COMP

GND