AN4394 datasheet(13/18 Pages) STMICROELECTRONICS

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AN4394

Component selection

4.3

Capacitance on MPP-REF pin

It's recommended to use C8 = 10 nF in most of the application cases.

4.3.1

Inductor selection

The SPV1050 device controls the switching of the integrated DC-DC by limiting the peak

current flowing through the inductor L1.

L1 = 22 µH covers the most typical application range: the lower is the series resistance of

the selected inductor, the lower is its DC loss. The current capability of the selected inductor

must be ≥ 200 mA.

4.3.2

Output voltage ripple

In case of battery with high series resistance and a fast load transient, the capacitor on the

STORE pin may momentarily discharge and cause the undesired triggering of the VUVP

threshold, implying the battery disconnection.

Although the fast transient might be masked by a proper capacitance between the UVP and

GND pins, if the battery has a low peak current capability, the voltage on the STORE pin

may further drop down lower than VUVP.

Increasing the capacitance on the STORE pin has the drawback of affecting the output time

constant and consequently delays the startup time. The same capacitor might be placed in

parallel to the battery, on the BATT pin.

The selection of the battery and of the output capacitance on the STORE pin (C9) is strictly

related to the following application parameters:



The series resistance of the battery (RBATT)



The EOC threshold (VEOC) and the UVP threshold (VUVP)



The maximum load current (ILOAD(MAX))



The TLOAD(ON), how long the load sink ILOAD(MAX)



The maximum allowed voltage drop on LDOs outputs (VDROP(MAX))

The maximum current that can be supplied to the load is the sum of the currents that can be

supplied by the battery (IBATT(MAX)) and by the C9 (ISTORE):

Equation 7

ILOAD(MAX) = IBATT(MAX) + ISTORE

The maximum current that the battery can supply without triggering the UVP threshold is:

Equation 8

The amount of charge that the C9 can supply is:

Equation 9

Q9 = C9

• VDROP(MAX)

IBATT MAX



VBATT VUVP

–

RBATT

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