MPM3805GQB-18-P Datasheet, PDF (13/16 Pages) MPS Industries, Inc. – 6V Input, 0.6A Module Synchronous Step-Down Converter with Integrated Inductor

MPM3805 –6V, 0.6A SIMPLE MODULE WITH INTEGRATED INDUCTOR
APPLICATION INFORMATION
COMPONENT SELECTION
Setting the Output Voltage
The external resistor divider is used to set the
output voltage (see Typical Application on page
16). The feedback resistor R1 cannot be too
large or too small considering the trade-off for
stability and dynamics. Choose R1 between
40kΩ to 80kΩ. R2 is given by:
R2 =
R1
V_{out − 1}
0.6
The feedback circuit is shown in Figure 2.
MPM3805
FB
OUT
R1
R2
Figure 2: Feedback Network
Table 1 lists the recommended resistor values for
common output voltages.
Table 1: Resistor Values for Common Output
Voltages
V_OUT (V)
1.0
1.2
1.8
2.5
3.3
R1 (kΩ)
40(1%)
40(1%)
60(1%)
80(1%)
80(1%)
R2 (kΩ)
60(1%)
40(1%)
30(1%)
25(1%)
17.7(1%)
Selecting the Input Capacitor
The input current to the step-down converter is
discontinuous, therefore a capacitor is required to
supply the AC current while maintaining the DC input
voltage. For optimal performance, use low ESR
capacitors. Ceramic capacitors with X5R or X7R
dielectrics are highly recommended due to their
low ESR and small temperature coefficients. For
most applications, a 10µF capacitor is sufficient.
For higher output voltage, a 22µF may be
needed to enhance system stability.
Since the input capacitor absorbs the input
switching current it requires an adequate ripple
current rating. The RMS current in the input
capacitor can be estimated by:
I_C1 = I_LOAD ×
V_OUT
V_IN
×⎛⎜⎜⎜⎝1−
V_OUT
V_IN
⎞
⎟
⎟⎟⎠
The worst case condition occurs at V_IN = 2V_OUT,
where:
I_C1
=
I_LOAD
2
For simplification, choose the input capacitor that
has a RMS current rating greater than half of the
maximum load current.
The input capacitor can be electrolytic, tantalum
or ceramic. When using electrolytic or tantalum
capacitors, a small, high quality ceramic
capacitor (i.e. 0.1μF), should be placed as close
to the IC as possible. When using ceramic
capacitors, check that they have enough
capacitance to provide sufficient charge to
prevent an excessive voltage ripple at input. The
input-voltage ripple caused by capacitance is
estimated by:
ΔV_IN
=
I_{LOAD ×}
f_S × C1
V_OUT
VIN
× ^⎛⎜1−
⎝
V_OUT
V_IN
⎞
⎟
⎠
Selecting the Output Capacitor
The output capacitor (C_OUT) is required to
maintain the DC output voltage. Ceramic
capacitors are recommended. Low ESR
capacitors are preferred to keep the output-
voltage ripple low. The output voltage ripple is
estimated by:
ΔV_OUT
=
V_OUT
f_S × L₁
×
⎛
⎜¹
−
⎝
V_OUT
V_IN
⎞⎛
⎟ ^× ⎜^RESR
⎠⎝
+
1⎞
8
×
f_S
×
C2
⎟
⎠
Where L₁ is the inductor value, and R_ESR is the
equivalent series resistance (ESR) value of the
output capacitor (L₁ is 0.47µH).
When using ceramic capacitors, the impedance
at the switching frequency is dominated by the
MPM3805 Rev.1.11
www.MonolithicPower.com
13
3/23/2015
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