AN2390 datasheet(10/42 Pages) STMICROELECTRONICS

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Evaluation board implementation

AN2390

10/42

2.1.1

DC (Buck-Boost) converter component selection

The value of inductor L and capacitor C are selected by the following section. For more

detail please refer to AN2389.

Inductor selection

The minimum value of the inductor can be selected by choosing the maximum of the values

given by the following two formulae:

Here Vsat1 and Vsat2 are the saturation voltages of the two switches Sw1 and Sw2.

Iout and Vout are the maximum output current and voltage respectively.

Vd1 and Vd2 is the voltage drop across diodes d1 and d2.

The duty cycle of the PWM signals driving switch Sw1 (PWM1) and Sw2 (PWM2) are D1

and D2 respectively.

Capacitor selection

The minimum capacitor value can be selected using the following formula to keep the

variation in Vout with in 1%:

In practice we take inductor and capacitor values that are 25% more than the values

calculated using the above formulae.

2.1.2

Battery discharge protection

If the charger is not powered on or if the battery is already fully charged, the PNP transistor

is kept permanently off which isolates the battery from the charger. Because of series diode

available in the buck-boost circuitry there is no reverse current flowing into the charger.

Therefore, the battery discharges into the output capacitor and resistive bridge. This allows

battery voltage measurement while consuming very little current. Also some leakage current

flows through the output capacitor.

T * [ ( Vin - Vsat1) * D1 - Vsat2 * D2 - Vout * (D1 - D2))]

Lmin =

2 * Iout

T * [ Vd1 + Vout] * ( 1 - D1)

Lmin =

2 * Iout

100 * Iout * (1 - D1) * T

Cmin =

Vout

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