Datasheet

C
SS
=
I
SS
x t
SS
V
ref
C
IN
=
Iomax x D x (1-D)
f
s
x 'V
IN-MAX
LM3151, LM3152, LM3153
www.ti.com 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
• ΔV
IN-MAX
is the maximum allowable input ripple voltage
A good starting point for the input ripple voltage is 5% of V
IN
.
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 C
BYP
capacitor should be placed directly at the VIN pin. The recommended value is 0.1 µF.
7. Calculate Soft-Start Capacitor
where
• t
SS
is the soft-start time in seconds
• V
ref
= 0.6V
8. C
VCC
, and C
BST
and C
EN
C
VCC
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 C
VCC
. C
BST
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 C
BST
is
0.47 µF. The EN bypass capacitor, C
EN
, recommended value is 1000 pF when driving the EN pin from open
drain type of signal.
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