MP1497S – SYNCHRONOUS, STEP-DOWN CONVERTER WITH INTERNAL MOSFETS
APPLICATION INFORMATION
IL
Setting the Output Voltage
IL(MAX) ILOAD
2
The external resistor divider sets the output
voltage (see Typical Application on page 1).
The feedback resistor R1 sets the feedback
loop bandwidth. R2 is then given by:
Use a larger inductance for improved light-load
efficiency.
Selecting the Input Capacitor
R1
The input current to the step-down converter is
discontinuous, therefore requires a capacitor
supply the AC current to the step-down
converter while maintaining the DC input
voltage. Use low-ESR capacitors for the best
performance, such as ceramic capacitors with
X5R or X7R dielectrics that have low ESR and
small temperature coefficients. For most
applications, use a 22µF capacitor.
R2
V
OUT
1
0.807V
The T-type network shown in Figure 4 is highly
recommended.
R1
RT
8
FB
VOUT
R2
The input capacitor (C1) requires an adequate
ripple current rating because it absorbs the
input switching current. Estimate the RMS
current in the input capacitor as:
Figure 4: T-Type Network
Table 1 lists the recommended T-type resistors
value for common output voltages.
VOUT
VIN
VOUT
VIN
IC1 ILOAD
1
Table 1: Resistor Values for Common Output
Voltages
V
OUT (V)
1.0
1.2
1.8
2.5
3.3
5
R1 (kΩ)
20.5
R2 (kΩ)
82
Rt (kΩ)
82
The worst-case condition occurs at VIN=2VOUT
where:
,
30.1
60.4
32.4
19.1
13
82
ILOAD
IC1
40.2
56
2
40.2
33
For simplification, choose the input capacitor
with an RMS current rating greater than half of
the maximum load current.
40.2
33
40.2
7.68
33
The input capacitor can be electrolytic, tantalum
or ceramic. When using electrolytic or tantalum
Selecting the Inductor
Use a 1µH-to-10µH inductor with a DC current
rating of at least 25% percent higher than the
maximum load current for most applications.
For highest efficiency, select an inductor with a
DC resistance less than 15mꢀ. For most
designs, calculate the inductance value with:
capacitors, include
a
small, high-quality,
ceramic capacitor—e.g. 0.1μF—as close to the
IC as possible. When using ceramic capacitors,
make sure that they have enough capacitance
to prevent excessive input voltage ripple.
Estimate the input voltage ripple caused by the
capacitance as:
VOUT (V VOUT
)
IN
L1
ILOAD
VOUT
VOUT
V IL fOSC
V
1
IN
IN
fS C1
V
IN
V
IN
Where ΔIL is the inductor ripple current.
Selecting the Output Capacitor
Choose an inductor ripple current to be
approximately 30% of the maximum load
current. The maximum inductor peak current is:
The output capacitor (C2) maintains the DC
output voltage. Use ceramic, tantalum, or low-
ESR electrolytic capacitors. Low ESR
capacitors are preferred to keep the output
MP1497S Rev. 1.01
9/7/2015
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11
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