SLUS456F April 1999 – July 2018 UCC2808A-1 , UCC2808A-2 , UCC3808A-1 , UCC3808A-2
PRODUCTION DATA.
Refer to the PDF data sheet for device specific package drawings
| PIN | TYPE(1) | DESCRIPTION | ||
|---|---|---|---|---|
| NAME | SOIC, PDIP | TSSOP | ||
| COMP | 1 | 3 | O | COMP is the output of the error amplifier and the input of the PWM comparator. The error amplifier in the UCCx808A 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 UCCx808A family features built-in full-cycle soft start. Soft start is implemented as a clamp on the maximum COMP voltage. |
| CS | 3 | 5 | I | The input to the PWM, peak current, and overcurrent comparators. The overcurrent comparator is only intended for fault sensing. Exceeding the overcurrent threshold causes a soft-start cycle. An internal MOSFET discharges the current sense filter capacitor to improve dynamic performance of the power converter. |
| FB | 2 | 4 | I | 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 | 5 | 7 | G | Reference ground and power ground for all functions. Because of high currents, and high-frequency operation of the UCC3808A, a low impedance circuit board ground plane is highly recommended. |
| OUTA | 7 | 1 | O | 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. |
| OUTB | 6 | 8 | O | 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 | 4 | 6 | O | The oscillator programming pin. The UCC3808A’s oscillator tracks VDD and GND internally, so that variations in power supply rails minimally affect frequency stability. Functional Block Diagrams 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 in Equation 1.
The recommended range of timing resistors is between 10 kΩ and 200 kΩ and range of timing capacitors is between 100 pF and 1000 pF. Timing resistors less than 10 kΩ must 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 | 8 | 2 | P | The power input connection for this device. Although quiescent VDD current is very low, total supply current is 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 Equation 2.
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. |