MP1495 – SYNCHRONOUS STEP-DOWN CONVERTER
APPLICATION INFORMATION
Setting the Output Voltage
Where ΔIL is the inductor ripple current.
The external resistor divider sets the output
voltage. The feedback resistor R1 also sets the
feedback loop bandwidth with the internal
compensation capacitor (see Typical Application
on page 1). Choose R1 around 40kꢀ, then R2 is:
Choose an inductor ripple current to be
approximately 30% of the maximum load current.
The maximum inductor peak current is:
ΔIL
2
IL(MAX) = ILOAD
+
R1
R2 =
Use a larger inductor for light-load conditions
(below 100mA) for improved efficiency.
V
OUT
− 1
0.807V
Setting the AAM Voltage
The AAM voltage sets the transition point from
AAM to CCM. Select a voltage that balances
efficiency, stability, ripple, and transient: A
relatively low AAM voltage improves stability and
ripple, but degrades transient and efficiency
during AAM mode; a relatively high AAM voltage
improves the transient and efficiency during AAM,
but degrades stability and ripple.
Use the T-type network when VOUT is low, as
shown in Figure 4.
R1
RT
8
FB
VOUT
R2
Figure 4: T-Type Network
AAM voltage is set from the tap of a resistor
divider from the VCC (5V) pin, as shown in Figure
5.
Table 1 lists the recommended T-type resistor
value for common output voltages.
VCC(5V)
Table 1: Resistor Selection for Common Output
Voltages
R3
V
OUT (V)
1.0
1.2
1.8
2.5
3.3
5
R1 (kΩ)
20.5(1%)
30.1(1%)
40.2(1%)
40.2(1%)
40.2(1%)
40.2(1%)
R2 (kΩ)
82(1%)
Rt (kΩ)
82(1%)
82(1%)
56(1%)
33(1%)
33(1%)
33(1%)
AAM
R4
60.4(1%)
32.4(1%)
19.1(1%)
13(1%)
Figure 5: AAM Network
Generally, choose R4 to be around 10kꢀ, then
R3 is:
7.68(1%)
Selecting the Inductor
VCC
AAM
⎛
⎜
⎞
⎠
R3 = R4
−1
⎟
⎝
For most applications, use a 1µH-to-10µH
inductor with a DC current rating that is at least
25% percent higher than the maximum load
current. Select an inductor with a DC resistance
less than 15mꢀ for highest efficiency. For most
designs, the inductance value can be derived
from the following equation.
VOUT ×(V − VOUT
)
IN
L1 =
V × ΔIL × fOSC
IN
MP1495 Rev. 1.04
12/26/2012
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