MIC2164_10 Datasheet, PDF (21/39 Pages) Micrel Semiconductor – Synchronous Buck Controllers Featuring Adaptive On-Time Control 28V Input, Constant Frequency

Micrel, Inc.
Setting Output Voltage
The MIC2164/-2/-3 requires two resistors to set the
output voltage, as shown in Figure 6:
MIC2164/-2/-3/C
In addition to the external ripple injection added at the
FB pin, internal ripple injection is added at the inverting
input of the comparator inside the MIC2164/-2/-3, as
shown in Figure 7. The inverting input voltage V_INJ is
clamped to 1.2V. As V_OUT is increased, the swing of V_INJ
will be clamped. The clamped V_INJ reduces the line
regulation because it is reflected back as a DC error on
the FB terminal. Therefore, the maximum output voltage
of the MIC2164/-2/-3 should be limited to 5.5V to avoid
this problem. If a higher output voltage is required, use
the MIC2176 instead.
Figure 6. Voltage-Divider Configuration
The output voltage is determined by the equation:
V_OUT
=
V_REF
× (1 +
R1)
R2
(33)
where V_REF = 0.8V. A typical value of R1 can be between
3kΩ and 10kΩ. If R1 is too large, it may allow noise to be
introduced into the voltage feedback loop. If R1 is too
small, it will decrease the efficiency of the power supply,
especially at light loads. Once R1 is selected, R2 can be
calculated using:
R2 = ^{VREF × R1}
(34)
V_OUT − V_REF
Figure 7. Internal Ripple Injection
September 2010
21
M9999-091310-D