MP1522 – 25V STEP-UP CONVERTER IN A SOT23-5 PACKAGE
Variable Frequency
Constant-Peak-Current Operation
When the power MOSFET is turned on the
inductor current increases until the 0.45A
current limit is reached. The Power MOSFET is
then turned off for a minimum of 550ns. At the
end of this 550ns transition if the feedback pin
is still lower than the 1.23V internal reference
the power MOSFET will again be turned on,
otherwise the MP1522 waits until the voltage
drops below the threshold before turning on the
MOSFET again. This process allows for optimal
use of the inductor, while minimizing the output
ripple and size of the output capacitor and
maintaining low operating current.
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:
V
OUT
R1
+
R2
=
V
FB
R1
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 V
IN
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 bypass capacitor is typically 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 450mA 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 550ns or longer.
The minimum value inductor for this criterion is:
L
≥
1.31
×
10
−
6
×
V
IN
(
µ
H
)
Solving for V
OUT
⎛
R1
+
R2
⎞
V
OUT
=
V
FB
⎜
⎟
⎝
R1
⎠
Where V
OUT
is the output voltage, and V
FB
is the
1.23V feedback threshold. Substituting the
feedback threshold voltage:
⎛
R1
+
R2
⎞
V
OUT
=
1.23 V
⎜
⎟
⎝
R1
⎠
R1 can range from 10kΩ to 200kΩ. R2 is
determined the equation:
⎡⎛
V
R2
=
R1
⎢⎜
OUT
⎜
⎢⎝
V
FB
⎣
⎞ ⎤
⎟ −
1
⎥
⎟
⎠ ⎥
⎦
For example for an output voltage of 20V and
R1 = 130kΩ the formula yields:
⎡⎛
20
⎞ ⎤
R2
=
130k
Ω ⎢⎜
⎟ −
1
⎥
⎣⎝
1.23 V
⎠ ⎦
R2 = 2MΩ 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 1nF-10nF.
MP1522 Rev. 1.9
12/9/2005
For maximum V
IN
= 3.3V, use an inductor value
greater than 4.7µH, and for maximum V
IN
= 5V
use an inductor value greater than 6.8µH.
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