TM
MP1423 – 3A, 23V, 385KHz STEP-DOWN CONVERTER
Output Rectifier Diode
The input capacitor can be electrolytic, tantalum
or ceramic. When using electrolytic or tantalum
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.
capacitors,
a
small, high quality ceramic
capacitor, i.e. 0.1µF, should be placed as close to
the IC as possible.
Choose a diode whose maximum reverse voltage
rating is greater than the maximum input voltage,
and whose current rating is greater than the
maximum load current. Table 2 lists example
Schottky diodes and manufacturers.
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:
Table 2—Diode Selection Guide
⎛
⎜
⎝
⎞
⎟
⎟
⎠
ILOAD
VOUT
VIN
VOUT
⎜
∆V
=
×
× 1−
IN
fS × C1
V
IN
Voltage/Current Manufacture
Diode
Rating
Output Capacitor
SK33
SK34
30V, 3A
40V, 3A
30V, 3A
40V, 3A
30V, 3A
40V, 3A
Diodes Inc.
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:
Diodes Inc.
B330
Diodes Inc.
B340
Diodes Inc.
MBRS330
MBRS340
On Semiconductor
On Semiconductor
⎛
⎜
⎝
⎞
⎟
⎟
⎛
⎜
⎝
⎞
⎟
⎟
⎠
VOUT
VOUT
VIN
1
⎜
⎜
∆VOUT
=
× 1−
× RESR
+
Input Capacitor
fS × L
8 × fS × C2
⎠
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.
Where L is the inductor value, C2 is the output
capacitance value, and RESR 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:
Since the input capacitor (C1) absorbs the input
switching current it requires an adequate ripple
current rating. The RMS current in the input
capacitor can be estimated by:
⎛
⎞
⎟
⎟
⎠
VOUT
VOUT
⎜
∆VOUT
=
× 1−
8 × fS2 × L × C2
⎜
⎝
V
IN
⎛
⎞
⎟
VOUT
VIN
VOUT
VIN
⎜
IC1 = ILOAD
×
× 1−
⎜
⎝
⎟
⎠
In the case of 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
=
VOUT
VOUT
⎛
⎞
∆VOUT
=
× ⎜1−
⎟ ×RESR
2
⎜
⎟
fS ×L
VIN
⎝
⎠
For simplification, choose the input capacitor
whose RMS current rating greater than half of
the maximum load current.
The characteristics of the output capacitor also
affect the stability of the regulation system. The
MP1423 can be optimized for a wide range of
capacitance and ESR values.
MP1423 Rev. 1.1
1/6/2006
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