TM
MP2351 – 1.5A, 23V, 1.4MHz STEP-DOWN CONVERTER
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:
∆
V
IN
=
⎛
I
LOAD
V
V
×
OUT
× ⎜
1
−
OUT
f
S
×
C1 V
IN
⎜
V
IN
⎝
⎞
⎟
⎟
⎠
Compensation Components
The MP2351 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.
The DC gain of the voltage feedback loop is:
A
VDC
=
R
LOAD
×
G
CS
×
A
VEA
×
V
FB
V
OUT
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:
∆
V
OUT
=
V
OUT
⎛
V
× ⎜
1
−
OUT
f
S
×
L
⎜
V
IN
⎝
⎞
⎞ ⎛
1
⎟
⎟ × ⎜
R
ESR
+
⎟ ⎜
8
×
f
S
×
C2
⎟
⎠ ⎝
⎠
Where A
VEA
is the error amplifier voltage gain,
G
CS
is the current sense transconductance and
R
LOAD
is the load resistor value.
The system has two poles of importance. One
is due to the compensation capacitor (C3) and
the output resistor of error amplifier, and the
other is due to the output capacitor and the load
resistor. These poles are located at:
f
P1
=
f
P2
=
G
EA
2
π ×
C3
×
A
VEA
1
2
π ×
C2
×
R
LOAD
Where R
ESR
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
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:
∆V
OUT
=
V
OUT
8
×
f
S
2
Where
G
EA
is
transconductance.
the
error
amplifier
⎛
V
× ⎜
1
−
OUT
⎜
V
IN
×
L
×
C2
⎝
⎞
⎟
⎟
⎠
The system has one zero of importance, due to
the compensation capacitor (C3) and the
compensation resistor (R3). This zero is located
at:
f
Z1
=
1
2
π ×
C3
×
R3
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:
∆V
OUT
=
V
OUT
⎛
V
× ⎜
1
−
OUT
f
S
×
L
⎜
V
IN
⎝
⎞
⎟ ×
R
ESR
⎟
⎠
The system may have another zero of
importance, if the output capacitor has a large
capacitance and/or a high ESR value. The zero,
due to the ESR and capacitance of the output
capacitor, is located at:
f
ESR
=
1
2
π ×
C2
×
R
ESR
The characteristics of the output capacitor also
affect the stability of the regulation system. The
MP2351 can be optimized for a wide range of
capacitance and ESR values.
MP2351 Rev. 1.5
1/6/2006
www.MonolithicPower.com
MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited.
© 2006 MPS. All Rights Reserved.
6
MPS [ MONOLITHIC POWER SYSTEMS ]
