MP2351DQ MPS [Monolithic Power Systems], MP2351DQ Datasheet - Page 6

• Manufacturer Part Number: MP2351DQ
• Description: 1.5A, 23V, 1.4MHz Step-Down Converter
• Manufacturer: MPS [Monolithic Power Systems]

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:
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:
Where R
(ESR) value of the output capacitor and C2 is
the output capacitance value.
In
impedance at the switching frequency is
dominated by the capacitance. The output
voltage
capacitance. For simplification, the output
voltage ripple can be estimated by:
In
capacitors, the ESR dominates the impedance
at the switching frequency. For simplification,
the output ripple can be approximated to:
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
∆
V
OUT
the
the
∆V
=
∆
∆V
ESR
f
V
ripple
case
OUT
case
V
S
OUT
IN
OUT
×
is the equivalent series resistance
TM
=
L
=
×
f
=
I
S
8
LOAD
⎛
⎜ ⎜
⎝
of
×
f
V
1
×
of
is
S
OUT
C
−
f
×
S
1
V
V
L
2
×
ceramic
V
OUT
mainly
tantalum
OUT
MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited.
×
×
IN
V
L
⎛
⎜ ⎜
⎝
V
OUT
1
×
IN
⎞
⎟ ⎟
⎠
−
C
×
V
2
×
⎛
⎜
⎜
⎝
V
OUT
R
⎛
⎜ ⎜
⎝
×
IN
1
caused
ESR
⎛
⎜ ⎜
⎝
−
capacitors,
1
⎞
⎟ ⎟
⎠
or
−
×
V
V
OUT
+
R
V
IN
V
ESR
OUT
8
IN
electrolytic
×
⎞
⎟ ⎟
⎠
© 2006 MPS. All Rights Reserved.
f
by
⎞
⎟ ⎟
⎠
S
1
×
www.MonolithicPower.com
C
MP2351 – 1.5A, 23V, 1.4MHz STEP-DOWN CONVERTER
2
the
the
⎞
⎟
⎟
⎠
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:
Where A
G
R
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:
Where
transconductance.
The system has one zero of importance, due to
the compensation capacitor (C3) and the
compensation resistor (R3). This zero is located
at:
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:
LOAD
CS
is the current sense transconductance and
is the load resistor value.
A
VEA
VDC
G
EA
is the error amplifier voltage gain,
=
f
f
P
ESR
f
R
P
2
f
1
LOAD
Z
=
1
=
is
=
=
2
2
2
π
π
×
2
π
×
π
×
G
×
×
C
C
the
CS
G
C
C
2
3
1
EA
1
2
3
×
×
1
×
×
R
×
A
A
R
R
LOAD
VEA
VEA
ESR
error
3
×
V
V
OUT
FB
amplifier
6