L5963 datasheet(31/49 Pages) STMICROELECTRONICS

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L5963

Application information

7.3

Input capacitors (CIN)

The input capacitors must be chosen to support the maximum input operating voltage and

the maximum RMS input current required by the device.

The input capacitors must deliver the RMS current according to below equation:

Equation 7

Where Io is the maximum DC output current and D is the duty cycle. This function has a

maximum at D = 0.5 and it is equal to Io/2.

Equation 8

Vdl is the voltage drop across the low side DMOS, and Vdh is the voltage drop across the

high side DMOS.

For example, with 20% duty cycle, the input/output current multiplier is 0.400. Therefore, if

the regulator is delivering 3 A of steady-state load current, the input capacitor(s) must

support 0.400 × 3 A or 1.2 Arms.

Ceramic capacitors can deliver quite a bit of current but their total capacitance is relatively

low. Electrolytic capacitors typically offer much more capacitance than ceramic capacitors,

but can typically deliver a current of 100 to 500 mArms. So a good design will employ both

types of capacitors with the ceramic capacitors placed closest to the input pins of the device.

As a result, ceramic capacitors which have very low ESR and inductance are the best for

filtering the high frequency switching noise, and electrolytic capacitors are typically able to

provide more current over extended periods of time where VIN would otherwise droop.

7.4

Bootstrap capacitor (CBOOT)

A bootstrap capacitor must be connected between the BOOT and SW pins to provide

floating gate drive to the high-side MOSFET. For most applications 47 nF is sufficient. This

should be a ceramic capacitor with a voltage rating of at least 6 V.

7.5

Compensation network

The compensation network has to assure stability and good dynamic performance. The loop

of the device is based on the voltage mode control, compatible with TYPE III compensation

network (Figure 14). The error amplifier is a voltage operational amplifier with large

bandwidth, which is much larger than the closed-loop one.

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