UCC25705, UCC25706, UCC35705, UCC35706

HIGHSPEED VOLTAGE MODE PULSE WIDTH MODULATOR

SLUS473A – NOVEMBER 1999 – REVISED MARCH 2001

4

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pin descriptions

DISCH: A resistor to VIN sets the oscillator discharge current programming a maximum duty cycle. When grounded,

an internal comparator switches the oscillator to a quick discharge mode. A small 100-pF capacitor between DISCH

and 250

FB: Input to the PWM comparator. This pin is intended to interface with an optocoupler. Input impedance is 50-k

Ω

typical.

GND: Ground return pin.

ILIM: Provides a pulse-by-pulse current limit by terminating the PWM pulse when the input is above 200 mV. This

provides a high speed (25 ns typical) path to reset the PWM latch, allowing for a pulse-by-pulse current limit.

OUT: The output is intended to drive an external FET driver or other high impedance circuits, but is not intended to

directly drive a power MOSFET. This improves the controller’s noise immunity. The output resistance of the PWM

controller, typically 60

Ω pull-up and 30 Ω pull-down, will result in excessive rise and fall times if a power MOSFET

is directly driven at the speeds for which the UCC35705/6 is optimized.

RC: The oscillator can be configured to provide a maximum duty cycle clamp. In this mode the on–time is set by RT

and CT, while the off-time is set by RDISCH and CT. Since the voltage ramp on CT is proportional to VIN, feed-forward

action is obtained. Since the peak oscillator voltage is also proportional to VIN, constant frequency operation is

maintained over the full power supply input range. When the DISCH pin is grounded, the duty cycle clamp is disabled.

The RC pin then provides a low impedance path to ground CT during the off time.

VDD: Power supply pin. This pin should be bypassed with a 0.1-

µF capacitor for proper operation. The undervoltage

lockout function of the UCC35705/6 allows for a low current startup mode and ensures that all circuits become active

in a known state. The UVLO thresholds on the UCC35705 are appropriate for a dc-to-dc converter application. The

wider UVLO hysteresis of the UCC35706 (typically 4 V) is optimized for a bootstrap startup mode from a high

impedance source.

VFF: The feed-forward pin provides the controller with a voltage proportional to the power supply input voltage. When

the oscillator is providing a duty cycle clamp, a current of 2

RFF between VFF and GND then set VFF to:

VFF

[ VIN

2

R

FF

2

R

When the DISCH pin is grounded and the duty cycle clamp is not used, the internal current source is disabled and

a resistor divider from VIN is used to set VFF. In either case, when the voltage on VFF is less than 1.0 V, both the

output and oscillator are disabled.