Datasheet

'V
IN
=
I
OUT
4 x f
SW
x C
IN
=
= 636 mV
7A
4 x 250 kHz x 11 PF
C
O
=
('V + V
OUT
)
2
- V
OUT
2
'I
PP
I
O
+
2
L x
2
LM5088, LM5088-Q1
www.ti.com SNVS600H –DECEMBER 2008–REVISED MARCH 2013
OUTPUT CAPACITORS
The output capacitors smooth the inductor current ripple and provide a source of charge for load transient
conditions. The output capacitor selection is primarily dictated by the following specifications:
1. Steady-state output peak-peak ripple (ΔV
PK-PK
)
2. Output voltage deviation during transient condition (ΔV
Transient
)
For the 5V output design example, ΔV
PK-PK
= 50 mV (1% of VOUT) and ΔT
Transient
= 100 mV (2% of VOUT) was
chosen. The magnitude of output ripple primarily depends on ESR of the capacitors while load transient voltage
deviation depends both on the output capacitance and ESR.
When a full load is suddenly removed from the output, the output capacitor must be large enough to prevent the
inductor energy to raise the output voltage above the specified maximum voltage. In other words, the output
capacitor must be large enough to absorb the inductor’s maximum stored energy. Equating the stored energy
equations of both the inductor and the output capacitor it can be shown that:
(17)
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:
(18)
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.
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