MIC2166_1009 Datasheet, PDF (20/28 Pages) Micrel Semiconductor – Adaptive On-Time DC-DC Controller Hyper Speed Control™ Family

Micrel, Inc.
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 the equation (36):
K _div
=
ΔV_FB(PP)
V_IN
× ^{fSW × τ}
D × (1 − D)
(38)
Then the value of Rinj is obtained as:
R _inj
= (R1// R2) × ( ¹
K _div
− 1)
(39)
Step 3. Select C_inj as 100nF, which could be considered
as short for a wide range of the frequencies.
Setting Output Voltage
The MIC2166 requires two resistors to set the output
voltage, as shown in Figure 8.
MIC2166
The output voltage is determined by the equation:
V_OUT
=
V_REF
× (1+
R1)
R2
(40)
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}
(41)
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 MIC2166, 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
MIC2166 should be limited to 5.5V to avoid this problem.
Figure 8. Voltage-Divider Configuration
Figure 9. Internal Ripple Injection
September 2010
20
M9999-092410-C