ISL78840ASRHFSLASHPROTO datasheet(6/11 Pages) INTERSIL

6 / 11 page

FN6991.0

December 21, 2009

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 2.0MHz. The

charge time, tC, the discharge time, tD, the switching

frequency, f, and the maximum duty cycle, DMAX, can be

approximated from the Equations 1 through 4:

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 switching frequency for the

ISL78841, ISL78845ASRH. The value for the ISL78840,

ISL78843ASRH will be twice that shown in Figure 4.

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. This

GATE output is actively held low when VDD is below the

UVLO threshold.

VDD - VDD is the power connection for the device. The

total supply current will depend on the load applied to

OUT. Total IDD current is the sum of the operating

current and the average output current. Knowing the

operating frequency, f, and the MOSFET gate charge, Qg,

Typical Performance Curves

FIGURE 1. FREQUENCY vs TEMPERATURE

FIGURE 2. REFERENCE VOLTAGE vs TEMPERATURE

FIGURE 3. EA REFERENCE vs TEMPERATURE

FIGURE 4. RESISTANCE FOR CT CAPACITOR VALUES

GIVEN

-60

-40

-20

100 120 140

0.98

0.99

1.00

1.01

TEMPERATURE (°C)

-60

-40

-20

100

0.995

0.996

0.997

0.998

0.999

1.000

1.001

TEMPERATURE (°C)

140

120

-60

-40

-20

100 120 140

0.996

0.997

0.998

1.000

1.001

TEMPERATURE (°C)

110

100

103

RT (k

Ω)

100pF

220pF

330pF

470pF

1.0nF

2.2nF

3.3nF

4.7nF

6.8nF

0.533 RT CT

⋅⋅

≈

(EQ. 1)

–

CT In

0.008 RT

3.83

–

⋅

0.008 RT

1.71

–

⋅

---------------------------------------------⎠

⎞

⎝

⎛

⋅⋅

≈

(EQ. 2)

()

⁄

(EQ. 3)

C f

⋅

(EQ. 4)

ISL78840ASRH, ISL78841ASRH, ISL78843ASRH, ISL78845ASRH

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