CM3406 データシートの表示（PDF） - Monolithic Power Systems
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CM3406 Datasheet PDF : 8 Pages
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TM
Choose an inductor that will not saturate under
the maximum inductor peak current. The peak
inductor current can be calculated by:
I LP
= ILOAD
+
2
VOUT
× fS ×
L
×
⎜⎜⎝⎛1 −
VOUT
VIN
⎟⎟⎠⎞
Where ILOAD is the load current.
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 whose maximum reverse
voltage rating is greater than the maximum
input voltage, and whose current rating is
greater than the maximum load current.
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 (C1) absorbs the input
switching current it requires an adequate ripple
current rating. The RMS current in the input
capacitor can be estimated by:
IC1 = ILOAD ×
VOUT
VIN
×⎜⎜⎛1−
⎝
VOUT
VIN
⎟⎞
⎟
⎠
The worst-case condition occurs at VIN = 2VOUT,
where:
IC1
=
ILOAD
2
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. 0.1μF, 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 input. The
input voltage ripple caused by capacitance can
be estimated by:
ΔVIN
=
ILOAD ×
fS × C1
VOUT
VIN
× ⎜⎜⎝⎛1 −
VOUT
VIN
⎟⎟⎠⎞
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:
ΔVOUT
=
VOUT
fS × L
× ⎜⎜⎝⎛1 −
VOUT
VIN
⎟⎟⎠⎞ × ⎜⎜⎝⎛RESR
+
8
×
1
fS ×
C2
⎟⎟⎠⎞
Wh
ere RESR is the equivalent series resistance (ESR)
value of the output capacitor and C2 is the output
capacitance value.
In the case of ceramic capacitors, the output
voltage ripple is mainly caused by the
capacitance. For simplification, the output voltage
ripple can be estimated by:
ΔVOUT
=
8
×
VOUT
fS2 × L ×
C2
×
⎜⎜⎝⎛1 −
VOUT
VIN
⎟⎟⎠⎞
Where L is the inductor value.
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:
ΔVOUT
=
VOUT
fS × L
×
⎜⎜⎝⎛1 −
VOUT
VIN
⎟⎟⎠⎞ × RESR
The characteristics of the output capacitor also
affect the stability of the regulation system. The
CM3406 can be optimized for a wide range of
capacitance and ESR values.
Compensation Components
The CM3406 employs current mode control for
easy compensation and fast transient response.
The system stability and transient response are
controlled through the COMP pin. COMP pin is
the output of the internal transconductance
error amplifier. A series capacitor-resistor
combination sets a pole-zero combination to
control the characteristics of the control system.
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