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參數(shù)資料
| 型號： | EV0059 |
| 廠商： | Monolithic Power Systems, Inc. |
| 英文描述： | 1.2A Synchronous Rectified Step-Down Converter |
| 中文描述： | 1.2A的同步整流降壓轉換器 |
| 文件頁數(shù)： | 6/11頁 |
| 文件大?。?/td> | 309K |
| 代理商： | EV0059 |
MP1567 – 1.2A SYNCHRONOUS RECTIFIED STEP-DOWN CONVERTER
MP1567 Rev. 2.3
1/3/2006
www.MonolithicPower.com
6
MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited.
2006 MPS. All Rights Reserved.
TM
APPLICATION INFORMATION
COMPONENT SELECTION
Internal Low-Dropout Regulator
The internal power to the MP1567 is supplied
from the input voltage through an internal 2.4V
low-dropout linear regulator, whose output is
BP. Bypass BP to SGND with a 10nF or greater
capacitor to insure the MP1567 operates
properly. The internal regulator cannot supply
more current than is required to operate the
MP1567, therefore do not apply any external
load to BP.
Soft-Start
The MP1567 includes a soft-start timer that
slowly ramps the output voltage at startup to
prevent excessive current at the input. This
prevents premature termination of the battery
voltage at startup due to input current overshoot
at startup.
When power is applied to the MP1567 a 2
μ
A
internal current source charges the external
capacitor at SS. As the capacitor charges, the
voltage at SS will rise. The MP1567 internally
limits the feedback threshold voltage at FB to
that of the voltage at SS. This forces the output
voltage to rise at the same rate as the voltage
at SS, forcing the output voltage to ramp
linearly from 0V to the desired regulation
voltage during soft-start. The soft-start period is
determined by the equation:
5
C
45
.
t
SS
×
=
Where C5
(in nF) is the soft-start capacitor from
SS to GND, and t
SS
(in ms) is the soft-start
period. Determine the capacitor required for a
given soft-start period by the equation:
SS
t
22
.
5
C
×
=
Use values for C5 between 10nF and 22nF to
set the soft-start period (between 4ms and
10ms).
Setting the Output Voltage
Set the output voltage by selecting the resistive
voltage divider ratio. The voltage divider drops
the output voltage to the 0.9V feedback
threshold voltage. Use 10k
for the low-side
resistor of the voltage divider. Determine the
high side resistor by the equation:
=
1
R
V
V
V
2
R
FB
FB
OUT
Where R2 is the high-side resistor, R1 is the
low-side resistor, V
OUT
is the output voltage and
V
FB
is the feedback regulation threshold.
For R1 = 10k
and V
FB
= 0.9V, then
R2(k
) = 11.1k
(V
OUT
– 0.9V)
Selecting the Input Capacitor
The input current to the step-down converter is
discontinuous, so a capacitor is required to
supply the AC current to the step-down
converter while maintaining the DC input
voltage. A low ESR capacitor is required to
keep the noise at the IC to a minimum. Ceramic
capacitors are preferred, but tantalum or low
ESR electrolytic capacitors will also suffice.
Use an input capacitor with a value greater than
10
μ
F. The capacitor can be electrolytic,
tantalum or ceramic. However, since it absorbs
the input switching current it requires an
adequate ripple current rating. Use a capacitor
with a RMS current rating greater than 1/2 of
the DC load current.
For insuring stable operation, place the input
capacitor as close to the IC as possible.
Alternately, a smaller high quality 0.1
μ
F
ceramic capacitor may be placed closer to the
IC with the larger capacitor placed further away.
If using this technique, it is recommended that
the larger capacitor be a tantalum or electrolytic
type. All ceramic capacitors should be placed
close to the MP1567.
Selecting the Output Capacitor
The output capacitor is required to maintain the
DC output voltage. Low ESR capacitors are
preferred to keep the output voltage ripple to a
minimum. The characteristics of the output
capacitor also affect the stability of the
regulation control system. Ceramic, tantalum or
low
ESR
electrolytic
recommended.
capacitors
are
In the case of ceramic capacitors, the
impedance at the switching frequency is
dominated by the capacitance, and so the
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