UCC3808A is a family of BICMOS push, high -speed, low power consumption, pulse width modulator. UCC3808A contains all control and drive circuits required for offline or DC-DC fixed frequency current mode switching power supply. The number of external components is the least.

The UCC3808A dual output drive level is arranged by push -pull configuration. Both outputs use the trigger to switch with half of the oscillator. According to the value of the timer capacitor and the resistor, the dead zone time between the two outputs is usually 60 ns to 200 NS, so the duty occupation ratio of each output level is limited to less than 50%.

Figure Figure

Explanation (continued)

The UCC3808A series provides a variety of packaging options, temperature range options, and under voltage locking level choose. This series has the UVLO threshold and lagging options for offline and battery power supply systems. The threshold is shown in the following table.

UCC3808A is an enhanced version of the UCC3808 series. The significant difference is that the A version has an internal discharge transistor from the CS tube to the ground. During the dead zone of the oscillator, the transistor is activated per clock cycle. In each cycle, any filter capacitor on the CS pin of this characteristic discharge helps minimize filter capacitors and current detection delays.

Pipefoot allocation

Company: COMP is the output of error amplifier and the input of PWM comparator. The error amplifier in the UCC3808A is a real low output impedance, 2 MBH -Hecheng computing amplifier. Therefore, the COMP foot can be both source current or exchange current. However, the internal current of the error amplifier is limited, so the zero occupation ratio can be forced by pulling the COMP to the GND.

The UCC3808A series has a built -in full -cycle soft startup function. Soft start is achieved as a clamp as the maximum compensation voltage.

Anti -terrorist elite: Input to PWM, peak current, and overcurrent comparator. Excessive current comparators are only used for fault detection. Excessive current threshold will cause a soft start cycle. Internal MOSFET drain -discharged fluid filter capacitance to improve the dynamic performance of the power converter.

Federal Survey Bureau: Reverse input of errors. In order to obtain the best stability, the length of the FB lead as much as possible and the smaller FB messy capacitance.

The ground: Reference ground and power ground for all functions. Due to the high current and high -frequency operations of UCC3808A, it is strongly recommended to use a low -impedance circuit board ground plane.

OUTA and OUTB: AC large current output level. Both stages can drive the grid of MOSFET. Each level can generate a 500 mAh peak source current and the peak exchange of 1 peaceflow.

In the push -pull structure, the output level is switched with half of the frequency of an oscillator. When the voltage on the RC pin increases, one of the two outputs is high, but during the decline, both outputs are closed. The dead time between the two outputs and the rising time of the slow output than the decrease time ensure that the two outputs cannot be opened at the same time. The time of the dead area is usually 60 ns to 200 ns, depending on the value of timer capacitors and resistors.

The large current output drive is composed of MOSFET output devices, and the output device is converted from VDD to GND. Each output level also provides a very low impedance to overwhelm down. This means that in many cases, there is no need for external sketch clamping diode.

Reinforced concrete: oscillator programming tube. The oscillator of UCC3808A tracks VDD and GND internally, so the change of the power orbital has the least effect on the frequency stability. Figure 1 shows the oscillator box diagram.

Only two components are required for oscillator programming: one resistor (connected to VDD and RC) and a capacitor (connected to RC and GND). The frequency of similar oscillator is determined by the following simple formulas:

Among them, the unit is Hertz, the resistor unit is Ohm, and the capacitor unit is Fara. The recommended timing resistor range is between 10 kΩ and 200 kΩ, and the range capacitor range is between 100 PF and 1000 PF. The timing resistor should be avoided by less than 10 kΩ.

In order to obtain the best performance, the time -time capacitor lead should be connected to GND as much as possible, connecting the VDD's timing transmission line to the VDD, and connecting the lead between the timing components and the RC to RC. Encourage separate grounding and VDD tracking of external timing networks.

Tube pin distribution (continued)

oscillator box diagram

Note a: oscillator produces jagged waves on RC. During the RC rising time, the output level was opened alternately, but both levels were closed within the decrease of RC. The output stage switch the frequency of 1/2 oscillator to ensure that the duty cycle of the two output is less than 50%.

Video display: The power input connection of this device. Although the static VDD current is very low, the total power supply will be higher, depending on the OUTA and OUTB current and the frequency of programming oscillator. The total VDD current is the sum of static VDD current and the sum of the average output current. Known the operating frequency and MOSFET gate charge (QG), the average output current can be calculated by the following formulas:

IOUT QG F, where F is the frequency

In order to prevent noise, the VDD widens To GND, use ceramic capacitors and electrolytic capacitors close to the chip as much as possible. It is recommended to use 1 μF decoupling capacitors.

Application information

200kHz push -pull application circuit with a full -wave rectifier is shown in Figure 2. Output, V, 5 V at the maximum 50 W, is isolated from the input power. Because UCC3808A is a peak current mode controller, the 2N2907 launch pole follows the placeder (buffer CT waveform) to provide a slope compensation required for the duty cycle to be greater than 50%. For a single -connected IC controller, the capacitor is very important. It is recommended to get as close to the IC 1 μF as possible. The controller power supply is a series RC used to start, and connects parallel with the bias winding in the output inductance used for steady -state operation.

The isolation is provided by a optocoupler, and the TL431 adjustable precision parallel adjuster is adjusted on the secondary side. The secondary side uses this part to achieve small signal compensation and strict voltage adjustment. According to cost, volume and mechanical strength, there are many choices for output inductors. There are several design schemes that are iron powder, molybdenum alloy (MPP) or iron oxygen magnetic core with air gap, as shown in the figure. The main power transformer has the core of the ER28 size of a magnetic company, which is made of P material to operate effectively at this frequency and temperature. The input voltage can be between 36 volt DC and 72 volts.

Application information