Datasheet

C
SS
=
1.205V
t
SS
x 11 PA
C
HB
t
'V
HB
Q
g
'V
IN
=
I
OUT
4 x f
SW
x C
IN
=
= 636 mV
7A
4 x 250 kHz x 11 PF
LM25088
LM25088-Q1
www.ti.com SNVS609H –DECEMBER 2008–REVISED MARCH 2013
Evaluating, the above equation with a ΔVout of 100 mV results in an output capacitance of 475 µF. As stated
earlier, the maximum peak to peak ripple primarily depends on the ESR of the output capacitor and the inductor
ripple current. To satisfy the ΔV
PK-PK
of 50 mV with 40% inductor current ripple, the ESR should be less than 15
mΩ. In this design example a 470 µF aluminum capacitor with an ESR of 10 mΩ is paralleled with two 47 µF
ceramic capacitors to further reduce the ESR.
INPUT CAPACITORS
The input power supply typically has large source impedance at the switching frequency. Good quality input
capacitors are necessary to limit the ripple voltage at the VIN pin while supplying most of the switch current
during the on-time. When the buck switch turns ON, the current into the external FET steps to the valley of the
inductor current waveform at turn-on, ramps up to the peak value, and then drops to zero at turn-off. The input
capacitors should be selected for RMS current rating and minimum ripple voltage. A good approximation for the
ripple current is I
RMS
> I
OUT
/2.
Quality ceramic capacitors with a low ESR should be selected for the input filter. To allow for capacitor
tolerances and voltage rating, five 2.2 µF, 100V ceramic capacitors were selected. With ceramic capacitors, the
input ripple voltage will be triangular and will peak at 50% duty cycle. Taking into account the capacitance
change with DC bias a worst case input peak-to-peak ripple voltage can be approximated as:
(17)
When the converter is connected to an input power source, a resonant circuit is formed by the line impedance
and the input capacitors. This can result in an overshoot at the VIN pin and could result in VIN exceeding its
absolute maximum rating. Because of those conditions, it is recommended that either an aluminum type
capacitor with an ESR or increasing CIN>10 x LIN While using aluminum type capacitor care should be taken to
not exceed its maximum ripple current rating. Tantalum capacitors must be avoided at the input as they are
prone to shorting.
VCC CAPACITOR
The capacitor at the VCC pin provides noise filtering and stability for the VCC regulator. The recommended value
should be no smaller than 0.1 µF, and should be a good quality, low ESR, ceramic capacitor. A value of 1 µF
was selected for this design.
BOOTSTRAP CAPACITOR
The bootstrap capacitor between HB and SW pins supplies the gate current to charge the high-side MOSFET
gate at each cycle’s turn-on as well as supplying the recovery charge for the bootstrap diode (D1).The peak
current can be several amperes. The recommended value of the bootstrap capacitor is at least 0.022 µF and
should be a good quality, low ESR, ceramic capacitor located close to the pins of the IC. The absolute minimum
value for the bootstrap capacitor is calculated as:
where
• Q
g
is the high-side MOSFET gate charge
• ΔV
HB
is the tolerable voltage droop on C
HB
, which is typically less than 5% of the VCC (18)
A value of 0.1 µF was selected for this design.
SOFT-START CAPACITOR
The capacitor at the SS capacitor determines the soft-start time, the output voltage to reach the final regulated
value. The value of C
SS
for a given time is determined from:
(19)
For this design example, a value of 0.022 µF was chosen for a soft start time of approximately 2 ms.
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