MIC2165_1011 Datasheet, PDF (21/28 Pages) Micrel Semiconductor – Adaptive On-Time DC-DC Controller Featuring HyperLight Load®

Micrel, Inc.
MIC2165
Figure 8b.
R2
R1 + R2
×
ΔV_OUT
<
20mV ^and
Δ V_OUT
> 20mV
Figure 9. Voltage-Divider Configuration
The output voltage is determined by the equation:
V_OUT
=
V_REF
× (1+
R1)
R2
(41)
Figure 8c. ΔV_OUT < 20mV
The process of sizing the ripple injection resistor and
capacitors is:
Step 1. Select C_ff to feed all output ripples into the
feedback pin and make sure the large time constant
assumption is satisfied. Typical choice of C_ff is 1nF to
100nF if R1 and R2 are in kΩ range.
Step 2. Select R_inj according to the expected feedback
voltage ripple. According to Equation 37:
K _div
=
ΔV_FB(PP)
V_IN
× ^{fSW × τ}
D × (1 − D)
(39)
Then the value of R_inj is obtained as:
where V_REF = 0.8V. 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. The total voltage divider
resistance R1+R2 is recommended to be 7.5kΩ. Once R1
is selected, R2 can be calculated using:
R2 = ^{VREF × R1}
(42)
V_OUT − V_REF
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 MIC2165, as shown in Figure
10. 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 MIC2165 should be
limited to 5.5V to avoid this problem.
R _inj
= (R1// R2) × ( ¹
K _div
− 1)
(40)
Step 3. Select C_inj as 100nF, which could be considered
as short for a wide range of the frequencies.
Setting Output Voltage
The MIC2165 requires two resistors to set the output
voltage, as shown in Figure 9.
September 2010
Figure 10. Internal Ripple Injection
21
M9999-092410-E