LM3578AM datasheet(14/30 Pages) TI

14 / 30 page

LM2578A, LM3578A

SNVS767E – AUGUST 2000 – REVISED APRIL 2013

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BUCK REGULATOR

The buck configuration is used to step an input voltage down to a lower level. Transistor Q1 in Figure 27 chops

the input DC voltage into a squarewave. This squarewave is then converted back into a DC voltage of lower

magnitude by the low pass filter consisting of L1 and C1. The duty cycle, D, of the squarewave relates the output

voltage to the input voltage by the following equation:

Vout = D × Vin = Vin × (ton)/(ton + toff).

Figure 27. Basic Buck Regulator

Figure 28 is a 15V to 5V buck regulator with an output current, Io, of 350 mA. The circuit becomes discontinuous

at 20% of Io(max), has 10 mV of output voltage ripple, an efficiency of 75%, a load regulation of 30 mV (70 mA to

350 mA) and a line regulation of 10 mV (12

≤ Vin ≤ 18V).

Component values are selected as follows:

R1 = (Vo − 1) × R2 where R2 = 10 kΩ

R3 = V/Isw(max)

R3 = 0.15

where:

V is the current limit sense voltage, 0.11V

Isw(max) is the maximum allowable current thru the output transistor.

L1 is the inductor and may be found from the inductance calculation chart (Figure 29) as follows:

Given Vin = 15V

Vo = 5V

Io(max) = 350 mA

fOSC = 50 kHz

Discontinuous at 20% of Io(max).

Note that since the circuit will become discontinuous at 20% of Io(max), the load current must not be allowed to fall

below 70 mA.

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