MP1495 – SYNCHRONOUS STEP-DOWN CONVERTER
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
possible. When using ceramic capacitors, make
sure that they have enough capacitance to
provide sufficient charge to prevent excessive
voltage ripple at the input. The input voltage
ripple caused by capacitance can be estimated
by:
V
=3.3V
V
OUT
V
=2.5V
OUT
=5V
OUT
⎛
⎞
⎟
⎠
ILOAD
VOUT
VOUT
V
=1.8V
OUT
ΔV
=
×
× 1−
⎜
IN
fS ×C1
V
IN
V
IN
⎝
V
=1.05V
OUT
Selecting the Output Capacitor
The output capacitor (C2) maintains the DC
output voltage. Use ceramic, tantalum, or low-
ESR electrolytic capacitors. For best results, use
low-ESR capacitors to keep the output voltage
ripple low. The output voltage ripple can be
estimated by:
0
2
4
6
8
10
12
Figure 6: AAM Selection for Common Output
Voltages (VIN=4.5V to 16V)
Selecting the Input Capacitor
The input current to the step-down converter is
discontinuous and therefore requires a capacitor to
supply the AC current to the step-down converter
while maintaining the DC input voltage. Use low-ESR
capacitors for the best performance. For best results,
use ceramic capacitors with X5R or X7R
dielectrics because of their low ESR and small
temperature coefficients. Use a 22µF capacitor
for most applications.
⎛
⎞ ⎛
⎞
⎟
⎠
VOUT
VOUT
V
1
ΔVOUT
=
× 1−
× R
⎟ ⎜
+
⎜
ESR
fS ×L1
8× fS ×C2
⎝
IN ⎠ ⎝
Where L1 is the inductor value and RESR is the
equivalent series resistance (ESR) value of the
output capacitor.
For ceramic capacitors, the capacitance
dominates the impedance at the switching
frequency, and thus causes the majority of the
output voltage ripple. For simplification, the
output voltage ripple can be estimated by:
C1 requires an adequate ripple current rating since it
absorbs the input switching current. Estimate the
RMS current in the input capacitor with:
⎛
⎞
⎟
⎠
VOUT
8× fS2 ×L1 ×C2
VOUT
⎛
⎞
⎟
VOUT
VIN
VOUT
VIN
ΔVOUT
=
× 1−
⎜
⎜
IC1 = ILOAD
×
× 1−
V
⎜
⎝
⎟
⎠
⎝
IN
For tantalum or electrolytic capacitors, the ESR
dominates the impedance at the switching
frequency. For simplification, the output ripple
can be approximated to:
The worst case condition occurs at VIN = 2VOUT
where:
,
ILOAD
IC1
=
2
VOUT
VOUT
⎛
⎞
ΔVOUT
=
× 1−
×RESR
⎜
⎟
⎠
For simplification, choose an input capacitor
whose RMS current rating greater than half of the
maximum load current.
fS ×L1
V
IN
⎝
The characteristics of the output capacitor also
affect the stability of the regulation system. The
MP1495 can be optimized for a wide range of
capacitance and ESR values.
The input capacitor can be electrolytic, tantalum
or ceramic. When using electrolytic or tantalum
capacitors, place a small, high-quality ceramic
capacitor (e.g. 0.1μF) as close to the IC as
MP1495 Rev. 1.04
12/26/2012
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12
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