MP4459 – 1.5A, 4MHz, 36V STEP-DOWN CONVERTER
MP4459 Rev. 1.03 www.MonolithicPower.com 12
4/9/2014 MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.
© 2014 MPS. All Rights Reserved.
Output Rectifier Diode
The output rectifier diode supplies the current to
the inductor when the high-side switch is off. To
reduce losses due to the diode forward voltage
and recovery times, use a Schottky diode.
Choose a diode who’s maximum reverse
voltage rating is greater than the maximum
input voltage, and who’s current rating is
greater than the maximum load current. Table 3
lists example Schottky diodes and
manufacturers.
Table 3—Output Diodes
Manufacturer Part Number
Voltage
Rating
(V)
Current
Rating
(A)
Package
Diodes Inc. B240A-13-F 40V 2A SMA
Diodes Inc. B340A-13-F 40V 3A SMA
Central semi CMSH2-40M 40V 2A SMA
Central semi CMSH3-40MA 40V 3A SMA
Input Capacitor
The input current to the step-down converter is
discontinuous, therefore a capacitor is required
to supply the AC current to the step-down
converter while maintaining the DC input
voltage. Use low ESR capacitors for the best
performance. Ceramic capacitors are preferred,
but tantalum or low-ESR electrolytic capacitors
may also suffice. Since the input capacitor
absorbs the input switching current it requires
an adequate ripple current rating. The RMS
current in the input capacitor can be estimated
by:
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
×
−×=
IN
OUT
IN
OUT
LOAD1C
V
V
1
V
V
II
The worse case condition occurs at V
IN = 2VOUT,
where:
2
I
I
LOAD
1C
=
For simplification, choose the input capacitor
whose RMS current rating greater than half of
the maximum load current. The input capacitor
can be electrolytic, tantalum or ceramic. When
using electrolytic or tantalum capacitors, a small,
high quality ceramic capacitor, i.e. 0.1F,
should be placed as close to the IC as possible.
When using ceramic capacitors, make sure that
they have enough capacitance to provide
sufficient charge to prevent excessive voltage
ripple at input. The input voltage ripple caused
by capacitance can be estimated by:
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
−××
×
=Δ
IN
OUT
IN
OUT
S
LOAD
IN
V
V
1
V
V
1Cf
I
V
Where C
IN is the input capacitance value.
Output Capacitor
The output capacitor is required to maintain the
DC output voltage. Ceramic, tantalum, or low
ESR electrolytic capacitors are recommended.
Low ESR capacitors are preferred to keep the
output voltage ripple low. The output voltage
ripple can be estimated by:
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
××
+×
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
−×
×
=Δ
2Cf8
1
R
V
V
1
1Lf
V
V
S
ESR
IN
OUT
S
OUT
OUT
Where L is the inductor value, C
O is the output
capacitance value, and R
ESR is the equivalent
series resistance (ESR) value of the output
capacitor.
In the case of ceramic capacitors, the
impedance at the switching frequency is
dominated by the capacitance. The output
voltage ripple is mainly caused by the
capacitance. For simplification, the output
voltage ripple can be estimated by:
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
−×
×××
=
IN
OUT
2
S
OUT
OUT
V
V
1
2C1Lf8
V
V
