MPQ4560 Datasheet, PDF (14/21 Pages) Monolithic Power Systems – Industrial-Grade, 2A, 2MHz, 55V Step-Down Converter Available in AEC-Q100

MPQ4560 – 2A, 2MHz, 55V, STEP-DOWN CONVERTER
Input Capacitor
The input current to the step-down converter is
discontinuous and 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 resistances (ESR) for
the best performance. Ceramic capacitors are
best, but tantalum or low-ESR electrolytic
capacitors may also suffice.
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.
When using electrolytic or tantalum capacitors,
place a small, high-quality, ceramic capacitor
(0.1μF) 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
capacitance is approximately:
V_IN

I_{LOAD }
f_S  C1
V_OUT
VIN
 _{}1 
V_OUT
V_IN

Output Capacitor
The output capacitor (C2) maintains the DC
output voltage. Use ceramic, tantalum, or low-
ESR electrolytic capacitors. Low-ESR capacitors
are preferred to keep the output voltage ripple
low. The output voltage ripple can be estimated
as:
V_OUT

V_OUT
f_S  L

^{1 }
V_OUT
V_IN



^RESR


8

1
f_S 
C2

Where L is the inductor value and R_ESR is the
ESR value of the output capacitor.
For ceramic capacitors, the capacitance
dominates the impedance at the switching
frequency and contributes the most to the output
voltage ripple. For simplification, the output
voltage ripple can be estimated by:
ΔV_OUT

8
V_OUT
f_S2  L 
C2
 _{}1 
V_OUT
V_IN

For tantalum or electrolytic capacitors, the ESR
dominates the impedance at the switching
frequency. For simplification, the output ripple is
approximately:
ΔV_OUT

V_OUT
f_S  L

¹

V_OUT
VIN
 ^{ R}ESR
The characteristics of the output capacitor also
affect the stability of the regulation system. The
MPQ4560 can be optimized for a wide range of
capacitances and ESR values.
Compensation Components
MPQ4560 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 series capacitor-
resistor combination sets a pole-zero
combination to control the control system’s
characteristics. The DC gain of the voltage
feedback loop is:
A _VDC
 R_LOAD
 G_CS
 A _VEA

V_FB
V_OUT
Where
 A_VEA is the error-amplifier voltage gain,
400V/V;
 G_CS is the current-sense transconductance,
5.6A/V; and
 R_LOAD is the load resistor value.
The system has two important poles: One from
the compensation capacitor (C3) and the output
resistor of error amplifier, and the other due to
the output capacitor and the load resistor. These
poles are located at:
f_P1

G_EA
2 π C3 
A _VEA
f_{P 2}

1
2 π C 2  R_LOAD
Where, G_EA is the
transconductance, 120μA/V.
error-amplifier
The system has one important zero due to the
compensation capacitor and the compensation
resistor (R3). This zero is located at:
f_Z1

1
2 π C3 R3
MPQ4560 Rev. 1.1
www.MonolithicPower.com
14
3/29/2013
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