TM
MP2359 – 1.2A, 24V, 1.4MHz STEP-DOWN CONVERTER IN A TSOT23-6
APPLICATION INFORMATION
Selecting the Input Capacitor
Setting Output Voltage
The input capacitor reduces the surge current
drawn from the input supply and the switching
noise from the device. The input capacitor
impedance at the switching frequency should be
less than the input source impedance to prevent
high frequency switching current from passing
through the input. Ceramic capacitors with X5R or
X7R dielectrics are highly recommended because
of their low ESR and small temperature
coefficients. For most applications, a 4.7µF
capacitor is sufficient.
The external resistor divider is used to set the
output voltage (see the schematic on front
page). Table 1 shows a list of resistor selection
for common output voltages. The feedback
resistor R1 also sets the feedback loop
bandwidth with the internal compensation
capacitor (see Figure 1). R2 can be determined
by:
R1
VOUT
R2 =
− 1
0.81V
Selecting the Output Capacitor
Table 1—Resistor Selection for Common
Output Voltages
The output capacitor keeps the output voltage
ripple small and ensures feedback loop stability.
The output capacitor impedance should be low
at the switching frequency. Ceramic capacitors
with X5R or X7R dielectrics are recommended
for their low ESR characteristics. For most
applications, a 22µF ceramic capacitor will be
sufficient.
VOUT (V)
1.8
R1 (kΩ)
80.6 (1%)
49.9 (1%)
49.9 (1%)
49.9 (1%)
R2 (kΩ)
64.9 (1%)
23.7 (1%)
16.2 (1%)
9.53 (1%)
2.5
3.3
5
PC Board Layout
Selecting the Inductor
The high current paths (GND, IN and SW) should
be placed very close to the device with short,
direct and wide traces. The input capacitor needs
to be as close as possible to the IN and GND
pins. The external feedback resistors should be
placed next to the FB pin. Keep the switch node
traces short and away from the feedback network.
A 1µH to 10µH inductor with a DC current rating
of at least 25% percent higher than the
maximum load current is recommended for
most applications. For highest efficiency, the
inductor’s DC resistance should be less than
200mꢀ. Refer to Table 2 for suggested surface
mount inductors. For most designs, the required
inductance value can be derived from the
following equation.
External Bootstrap Diode
It is recommended that an external bootstrap
diode be added when the input voltage is no
greater than 5V or the 5V rail is available in the
system. This helps improve the efficiency of the
regulator. The bootstrap diode can be a low
cost one such as IN4148 or BAT54.
VOUT × (VIN − VOUT
VIN × ∆IL × fSW
)
L =
Where ∆IL is the inductor ripple current.
Choose the inductor ripple current to be 30% of
the maximum load current. The maximum
inductor peak current is calculated from:
5V (External) or
V
(4.5V to 5V)
IN
BS
∆IL
IL(MAX) = ILOAD
+
10nF
2
MP2359
SW
Under light load conditions below 100mA, a
larger inductance is recommended for improved
efficiency. See Table 2 for suggested inductors.
Figure 2—External Bootstrap Diode
Also note that the maximum recommended load
current is 1A if the duty cycle exceeds 35%.
This diode is also recommended for high duty cycle
VOUT
operation (when
>65%) applications.
VIN
MP2359 Rev. 1.1
6/23/2006
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