L4960 / 4962 Monolithic Integrated Switching Power Supply

Key words: L4960 / 4962 monolithic integrated switching power supply

L4960 / 4962 is a monolithic integrated switching power supply chip produced by ST company. The working principle and pin function of L4960 and l4962 are exactly the same, except for the packaging form and maximum output current value. L4960 adopts sip-7 package and can output 2.5A current; L4962 adopts dep-l6 package, and the maximum output current is 1.5A. 1 pin function

The pin arrangement of L4960 and l4962 is shown in Figure 1. The long lead on L4960 represents the rear pin and the short lead represents the front pin. The front and rear rows of pins are staggered from each other. The metal radiating plate on the plastic shell is communicated with the ground, and the plate is provided with screw holes for fixing on the large radiator. Among these pins of L4960 (l4962) (the pin number of l4962 is in brackets), pin 2 (10) is the feedback end, and part of the output voltage can be fed back to the error amplifier through the resistance divider (detection resistance). Pin 3 (11) is the compensation end, which is connected with the output end of the error amplifier. External resistance capacitance elements can be used to compensate the frequency of the error amplifier. The 5 (14) pin (RT / CT) is connected to the timing resistor and capacitor, which can be used to determine the switching frequency. Pin 6 (15) is the soft start pin î–„ the external soft start capacitor can protect the chip. Pin 4 (4, 5, 12, 13) is signal ground. Pins 1 (7) and 7 (2) are input and output pins respectively. In addition, pins 1, 3, 6, 8, 9 and 16 of l4962 are empty. 2 internal structure

The schematic block diagram of L4960 and l4962 is shown in Figure 2 (the pin number of l4962 is in brackets). Its internal functional circuits mainly include: 5.1V reference voltage source, error amplifier, sawtooth wave generator, PWM comparator, power output stage, soft start circuit, output current limiting protection circuit and chip overheating protection circuit.

The voltage divider î–„ composed of detection resistors R3 and R4 in Figure 2 can be used to adjust the output voltage v0. If V0 is directly fed back to pin 2 without voltage divider, V0 will output a fixed 5V voltage. In addition, a high-frequency filter capacitor can be connected in parallel at both ends of R1 and C3 as required. L is the energy storage inductor, C5 is the output filter capacitor, and VD3 is the freewheeling diode. 50. C5 and VD3 constitute a step-down output circuit.

The power pulse modulation signal is led out from pin 7. When the signal is at high level (the switching power tube is on), in addition to supplying power to the load, some electric energy is stored in L and C5, and VD3 is cut off. When the power pulse signal becomes low (the switching power tube is off), VD3 is turned on, so that the electric energy stored in L supplies power to the load through the circuit composed of VD3, so as to maintain the output voltage V0 unchanged.

The basic working process of L4960 and l4962 is: the output voltage V0 is sampled by R3 and R4, sent to the inverting input of the error amplifier, compared with the 5.1V reference voltage added to the in-phase input, and then the amplitude of the obtained error voltage VR is used to control the output pulse width of the PWM comparator, Finally, V0 remains unchanged through power amplification and step-down output circuit. If the pulse period is t and the high-level duration is m, the duty cycle:

Ｄ＝（ｍ／Ｔ）１００％

Set the power efficiency as η， Then the pulse amplitude VP output by the switching power tube = η 6. VI is the DC input voltage, and finally:

Ｖ０＝ η ＤＶＩ

So when η、 When VI is constant, the output voltage can be adjusted as long as the duty cycle D is changed. 3 application circuit

3.1 output voltage adjustable circuit

Fig. 3 shows a power supply circuit with adjustable output voltage composed of L4960. In the figure, the AC 220V voltage is reduced by 100VA power frequency transformer, and then the DC voltage VI is obtained after bridge rectification and filtering, which is used as the input voltage of L4960. When the output voltage is directly connected with pin 2 to form a closed loop, the voltage stabilizing value V0 is 5.1V (can be approximately regarded as 5V); When the output voltage forms a closed loop through the 2 pins of the voltage divider, the value of î–„ V0 depends on the voltage division ratio. The voltage divider is composed of sampling feedback resistors R3 and R4, so the output voltage V0 can be calculated by the following formula:

Ｖ０＝（Ｒ３＋Ｒ４） ＶＲＥＦ／Ｒ３

In fact, L4960 and l4962 can only output an adjustable voltage of 5.1 40V. But sometimes it is hoped that the output voltage can be adjusted from 0V, which cannot be realized by conventional methods. A simple and practical method is introduced below. First, a - 5V voltage is generated by 7905, and then connected with the four pins of L4960 to make vgnd = - 5V, so that the regulation range of output voltage becomes 30 35V. 3.2 extended output current circuit

Several l4960s can be used in parallel to expand the output current to improve the output power, but a synchronization signal should be added when used in parallel. The specific method is to short-circuit the clock terminal (5 pins) of each L4960 and share an oscillation circuit. An oscillation signal f is also used as the synchronization signal of each chip to realize the parallel synchronous output of each chip. Of course, the external synchronization signal can also be used.

The second method is to connect a high-power switch tube to expand the output current. This method can expand the output current of L4960 to 10A. The specific circuit is shown in Figure 4.