UCC2808-1, UCC2808-2, UCC3808-1, UCC3808-2

LOW POWER CURRENT MODE PUSH-PULL PWM

SLUS168D – APRIL 1999 – REVISED AUGUST 2002

5

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pin descriptions

COMP: COMP is the output of the error amplifier and the input of the PWM comparator. The error amplifier in the

UCC3808 is a true low-output impedance, 2-MHz operational amplifier. As such, the COMP pin can both source and

sink current. However, the error amplifier is internally current limited, so that zero duty cycle can be externally forced

by pulling COMP to GND.

The UCC3808 family features built-in full cycle soft start. Soft start is implemented as a clamp on the maximum COMP

voltage.

CS: The input to the PWM, peak current, and overcurrent comparators. The overcurrent comparator is only intended

for fault sensing. Exceeding the overcurrent threshold will cause a soft start cycle.

FB: The inverting input to the error amplifier. For best stability, keep FB lead length as short as possible and FB stray

capacitance as small as possible.

GND: Reference ground and power ground for all functions. Due to high currents, and high frequency operation of

the UCC3808, a low impedance circuit board ground plane is highly recommended.

OUTA and OUTB: Alternating high current output stages. Both stages are capable of driving the gate of a power

MOSFET. Each stage is capable of 500-mA peak source current, and 1-A peak sink current.

The output stages switch at half the oscillator frequency, in a push/pull configuration. When the voltage on the RC

pin is rising, one of the two outputs is high, but during fall time, both outputs are off. This dead time between the two

outputs, along with a slower output rise time than fall time, insures that the two outputs can not be on at the same

time. This dead time is typically 60 ns to 200 ns and depends upon the values of the timing capacitor and resistor.

The high-current output drivers consist of MOSFET output devices, which switch from VDD to GND. Each output

stage also provides a very low impedance to overshoot and undershoot. This means that in many cases, external

schottky clamp diodes are not required.

RC: The oscillator programming pin. The UCC3808’s oscillator tracks VDD and GND internally, so that variations in

power supply rails minimally affect frequency stability. Figure 1 shows the oscillator block diagram.

Only two components are required to program the oscillator: a resistor (tied to the VDD and RC), and a capacitor (tied

to the RC and GND). The approximate oscillator frequency is determined by the simple formula:

f

OSCILLATOR +

1.41

RC

where frequency is in hertz, resistance in ohms, and capacitance in farads. The recommended range of timing

resistors is between 10 k

Ω and 200 kΩ and range of timing capacitors is between 100 pF and 1000 pF. Timing resistors

less than 10 k

Ω should be avoided.

For best performance, keep the timing capacitor lead to GND as short as possible, the timing resistor lead from VDD

as short as possible, and the leads between timing components and RC as short as possible. Separate ground and

VDD traces to the external timing network are encouraged.

VDD: The power input connection for this device. Although quiescent VDD current is very low, total supply current

will be higher, depending on OUTA and OUTB current, and the programmed oscillator frequency. Total VDD current

is the sum of quiescent VDD current and the average OUT current. Knowing the operating frequency and the

MOSFET gate charge (Qg), average OUT current can be calculated from:

I

OUT +

Qg

F, where F is frequency

To prevent noise problems, bypass VDD to GND with a ceramic capacitor as close to the chip as possible along with

an electrolytic capacitor. A 1-

µF decoupling capacitor is recommended.