7

FN9238.0

October 19, 2005

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

maximum duty cycle, Dmax, can be approximated from the

following equations:

The formulae have increased error at higher frequencies due

to propagation delays. 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 0 to 1.0V and has

an internal offset of 100mV.

GND - GND is the power and small signal reference ground

for all functions.

Typical Performance Curves

FIGURE 1. FREQUENCY vs TEMPERATURE

FIGURE 2. REFERENCE VOLTAGE vs TEMPERATURE

FIGURE 3. EA REFERENCE vs TEMPERATURE

FIGURE 4. RTCT vs FREQUENCY

-60 -40 -20

0

20

40

60

80 100 120 140

0.98

0.99

1

1.01

TEMPERATURE (°C)

-60 -40 -20

0

20

40

60 80 100

0.995

0.996

0.997

0.998

0.999

1.000

1.001

TEMPERATURE (°C)

140

120

-60 -40 -20

0

20

40

60

80 100 120 140

0.996

0.997

0.998

1.000

1.001

TEMPERATURE (°C)

1

10

100

1

10

100

RT (k

Ω)

CT =

100pF

220pF

330pF

470pF

1.0nF

2.2nF

3.3nF

4.7nF

6.8nF

TC 0.533 RT CT

•

•

≈

(EQ. 1)

TD RT

–

CT

0.008 RT 3.83

–

•

0.008 RT 1.71

–

•

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





ln

•

•

≈

(EQ. 2)

f

1TC TD

+

()

⁄

=

(EQ. 3)

•

=

(EQ. 4)

ISL8843