MP4558 – 1A, 2MHz, 55V STEP-DOWN CONVERTER
Input Capacitor
frequency. For simplification, the output ripple
can be approximated as:
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 capacitors with low equivalent series
resistance (ESR) for the best performance.
Ceramic capacitors are preferred, but tantalum or
low-ESR electrolytic capacitors may also suffice.
VOUT
VOUT
ΔVOUT
1
RESR
fS L
VIN
The characteristics of the output capacitor also
affect the stability of the regulation system. The
MP4558 can be optimized for a wide range of
capacitances and ESR values.
For simplification, choose the input capacitor with
an RMS current rating greater than half of the
maximum load current. The input capacitor (C1)
can be electrolytic, tantalum or ceramic.
Compensation Components
MP4558 employs current-mode control for easy
compensation and fast transient response. The
COMP pin controls the system stability and
transient response—the COMP pin is the output
of the internal error amplifier. A capacitor-resistor
When using electrolytic or tantalum capacitors,
include a small, high-quality ceramic capacitor—
i.e. 0.1μF—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:
combination in series sets
a
pole-zero
combination to control the characteristics of the
control system. The DC gain of the voltage
feedback loop is given by:
VFB
AVDC RLOAD GCS AVEA
VOUT
ILOAD
VOUT
VOUT
VIN
1
Where
fS C1
VIN
V
IN
AVEA is the error amplifier voltage gain,
400V/V,
Output Capacitor
The output capacitor (C2) maintains the DC
output voltage. Use ceramic, tantalum, or low-
ESR electrolytic capacitors for best results. Low
ESR capacitors are preferred to keep the output
voltage ripple low. The output voltage ripple can
be estimated by:
GCS
is
the
current
sense
transconductance,5.6A/V, and
RLOAD is the load resistor value.
The system has two poles of importance: One is
caused by the compensation capacitor (C3) and
the output resistor of error amplifier; the other is
caused by the output capacitor and the load
resistor. These poles are located at:
VOUT
VOUT
VIN
1
RESR
VOUT
1
fS L
8 fS C2
Where L is the inductor value and RESR is the
ESR value of the output capacitor.
GEA
fP1
2πC3 AVEA
For 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:
1
fP2
2πC2RLOAD
Where,
transconductance, 120μA/V.
GEA
is
the
error
amplifier
VOUT
VOUT
ΔVOUT
1
The system has one zero of importance from C3
and the compensation resistor (R3). This zero is
located at:
8 fS2 L C2
V
IN
For tantalum or electrolytic capacitors, the ESR
dominates the impedance at the switching
1
fZ1
2πC3R3
MP4558 Rev. 1.01
10/28/2013
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