MP1593 – 3A, 28V, 385kHz STEP-DOWN CONVERTER
MP1593 Rev. 2.11 www.MonolithicPower.com 8
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Output Rectifier Diode
The output rectifier diode supplies current to the
inductor when the high-side switch is off. Use a
Schottky diode to reduce losses due to diode
forward voltage and recovery times.
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.
Table 2—Diode Selection Guide
Diode
oltage/Current
Rating
Manufacture
SK33 30V, 3A Diodes Inc.
SK34 40V, 3A Diodes Inc.
B330 30V, 3A Diodes Inc.
B340 40V, 3A Diodes Inc.
MBRS330 30V, 3A On Semiconductor
MBRS340 40V, 3A On Semiconductor
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
will also suffice.
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:
IN
OUT
IN
OUT
LOAD1C
V
V
1
V
V
II
The worst-case condition occurs at V
IN
= 2V
OUT
,
where:
2
I
I
LOAD
1C
For simplification, choose the input capacitor
whose RMS current rating is 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. 0.1μF) 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 the input.
The input voltage ripple caused by the
capacitance can be estimated by:
IN
OUT
IN
OUT
S
LOAD
IN
V
V
1
V
V
1Cf
I
V
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:
2Cf8
1
R
V
V
1
Lf
V
V
S
ESR
IN
OUT
S
OUT
OUT
Where L is the inductor value, C2 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, which is the
main cause of the output voltage ripple. For
simplification, the output voltage ripple can be
estimated by:
IN
OUT
2
S
OUT
OUT
V
V
1
2CLf8
V
ΔV
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:
ESR
IN
OUT
S
OUT
OUT
R
V
V
1
Lf
V
ΔV
The characteristics of the output capacitor also
affect the stability of the regulation system. The
MP1593 can be optimized for a wide range of
capacitance and ESR values.
