MP2307 – 3A, 23V, 340KHz SYNCHRONOUS RECTIFIED STEP-DOWN CONVERTER
MP2307 Rev. 1.9 www.MonolithicPower.com 8
5/28/2008 MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited.
© 2008 MPS. All Rights Reserved.
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:
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
×
−×=
IN
OUT
IN
OUT
LOAD1C
V
V
1
V
V
II
The worst-case condition occurs at V
IN
= 2V
OUT
,
where I
C1
= I
LOAD
/2. For simplification, use an
input capacitor with a 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 for low ESR capacitors can
be estimated by:
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
−××
×
=∆
IN
OUT
IN
OUT
S
LOAD
IN
V
V
1
V
V
f1C
I
V
Where C1 is the input capacitance value.
Output Capacitor
The output capacitor (C2) 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:
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
××
+×
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
−×
×
=∆
2Cf8
1
R
V
V
1
Lf
V
V
S
ESR
IN
OUT
S
OUT
OUT
Where C2 is the output capacitance value and
R
ESR
is the equivalent series resistance (ESR)
value of the output capacitor.
When using ceramic capacitors, the impedance
at the switching frequency is dominated by the
capacitance which is the main cause for the
output voltage ripple. For simplification, the
output voltage ripple can be estimated by:
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
−×
×××
=
IN
OUT
2
S
OUT
OUT
V
V
1
2CLf8
V
V
When using tantalum or electrolytic capacitors,
the ESR dominates the impedance at the
switching frequency. For simplification, the
output ripple can be approximated to:
ESR
IN
OUT
S
OUT
OUT
R
V
V
1
Lf
V
V ×
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
−×
×
=
The characteristics of the output capacitor also
affect the stability of the regulation system. The
MP2307 can be optimized for a wide range of
capacitance and ESR values.
Compensation Components
MP2307 employs current mode control for easy
compensation and fast transient response. The
system stability and transient response are
controlled through the COMP pin. COMP is the
output of the internal transconductance error
amplifier. A series capacitor-resistor
combination sets a pole-zero combination to
govern the characteristics of the control system.
The DC gain of the voltage feedback loop is
given by:
OUT
FB
EACSLOADVDC
V
V
AGRA ×××=
Where V
FB
is the feedback voltage (0.925V),
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 the error amplifier, and the
other is due to the output capacitor and the load
resistor. These poles are located at:
VEA
EA
1P
A3C2
G
f
××π
=
LOAD
2P
R2C2
1
f
××π
=
Where G
EA
is the error amplifier transconductance.
