MP1522(2003) Ver la hoja de datos (PDF) - Monolithic Power Systems
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MP1522 Datasheet PDF : 8 Pages
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Monolithic Power Systems
MP1522
25V Step Up Converter
in 3mm QFN Package
Functional Description Continued
Setting the Output Voltage
The output voltage is set by a resistive voltage
divider connecting the output voltage to FB. The
voltage divider reduces the output voltage down
by the ratio:
VOUT/ VFB = ( R1 + R2 ) / R1
Solving for VOUT:
VOUT = VFB [(R1 + R2) / R1]
Where VOUT is the output voltage, and VFB is
the 1.235V feedback threshold. Substituting
the feedback threshold voltage:
VOUT = 1.235V [(R1 + R2) / R1]
R1 can range from 10KΩ to 1MΩ; a typical
value for R1 is 100KΩ. R2 is determined the
equation:
R2 = R1 [(VOUT/VFB ) – 1]
For example for an output voltage of 12V and
R1 = 100KΩ the formula yields:
R2 = 100K [(12.00/1.235V) – 1]
R2 = 871.65KΩ or 872KΩ the closest standard
1% resistor value.
The feedback divider requires a feed-forward
capacitor connected in parallel with R2. This
capacitor is typically in the range of 22ρF.
On/Off Control
The EN input is used to turn the converter on
and off. A high input at EN turns the converter
on and a low level turns the converter off.
When EN is not being used, connect to the
input source for automatic startup. For inputs
greater than 6V Pull EN to VIN through a
100KΩ resistor.
Input Capacitor
To reduce noise, use a low ESR/ESL type
input capacitor. A 1µF surface mount ceramic
capacitor close to the input supply side of the
inductor minimizes EMI and ripple current at
the input source. See the section on printed
circuit board layout for reducing noise and
emissions. A capacitor is required to prevent
noise at the MP1522. This capacitor should
also be a low ESR/ESL type capacitor. Place a
0.1µF surface mount ceramic capacitor
between BIAS and GND as close to the
MP1522 as possible.
Output Capacitor
To minimize output voltage ripple and noise
use a low ESR/ESL type output capacitor.
Typically a 1µF surface mount ceramic
capacitor is sufficient. When using aluminum
electrolytics, tantalum or aluminum polymer
capacitors the ESR/ESL dominate the output
ripple. See the section on printed circuit board
layout.
Inductor
The inductor stores energy during the power
MOSFET switch on-time. This energy is
transferred to the output during the switch off-
time. Select an inductor whose peak-current
rating is greater than the 380mA peak current
limit of the MP1522. Lower value inductors
yield higher switching frequencies and thus
lower output ripple voltage. However, using
too low a value inductor will cause an increase
in peak current limit due to the delay of the
current-limit comparator. Choose an inductor
such that the peak current limit is reached in
500ns or longer. The minimum value inductor
for this criterion is:
L ≥ 1.31 x 10-6 * VIN (µH)
For maximum VIN = 3.3V, use an inductor value
greater than 4.7µH, and for maximum VIN = 5V
use an inductor value greater than 6.8µH.
MP1522 Rev 1.7_06/26/03
Monolithic Power Systems, Inc.
5
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