LM3151 datasheet(15/27 Pages) TI1

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CSS =

ISS x tSS

Vref

CIN =

Iomax x D x (1-D)

fs x 'VIN-MAX

LM3151, LM3152, LM3153

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SNVS562G – SEPTEMBER 2008 – REVISED MARCH 2011

Figure 14. Typical MOSFET Gate Charge Curve

See following design example for estimated power dissipation calculation.

6. Calculate Input Capacitance

The main parameters for the input capacitor are the voltage rating, which must be greater than or equal to the

maximum DC input voltage of the power supply, and its rms current rating. The maximum rms current is

approximately 50% of the maximum load current.

where

•

ΔVIN-MAX is the maximum allowable input ripple voltage

A good starting point for the input ripple voltage is 5% of VIN.

When using low ESR ceramic capacitors on the input of the LM3151/2/3 a resonant circuit can be formed with

the impedance of the input power supply and parasitic impedance of long leads/PCB traces to the LM3151/2/3

input capacitors. It is recommended to use a damping capacitor under these circumstances, such as aluminum

electrolytic that will prevent ringing on the input. The damping capacitor should be chosen to be approximately 5

times greater than the parallel ceramic capacitors combination. The total input capacitance should be greater

than 10 times the input inductance of the power supply leads/pcb trace. The damping capacitor should also be

chosen to handle its share of the rms input current which is shared proportionately with the parallel impedance of

the ceramic capacitors and aluminum electrolytic at the LM3151/2/3 switching frequency.

The CBYP capacitor should be placed directly at the VIN pin. The recommended value is 0.1 µF.

7. Calculate Soft-Start Capacitor

where

•

tSS is the soft-start time in seconds

•

Vref = 0.6V

8. CVCC, and CBST and CEN

CVCC should be placed directly at the VCC pin with a recommended value of 1 µF to 2.2 µF. For input voltage

ranges that include voltages below 8V a 1 µF capacitor must be used for CVCC. CBST creates a voltage used to

drive the gate of the high-side FET. It is charged during the SW off-time. The recommended value for CBST is

0.47 µF. The EN bypass capacitor, CEN, recommended value is 1000 pF when driving the EN pin from open

drain type of signal.

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