MIC2176-1 Datasheet, PDF (20/31 Pages) Micrel Semiconductor – Wide Input Voltage, Synchronous Buck Controllers Featuring Adaptive On-Time Control

Micrel, Inc.
MIC2176
The input capacitor must be rated for the input current
ripple. The RMS value of input capacitor current is
determined at the maximum output current. Assuming
the peak-to-peak inductor current ripple is low:
I_CIN(RMS) ≈ I_OUT(max) × D × (1− D)
(25)
The power dissipated in the input capacitor is:
P_DISS(CIN) = I_CIN(RMS)2 × ESR_CIN
(26)
Voltage Setting Components
The MIC2176 requires two resistors to set the output
voltage as shown in Figure 5:
less than 20mV. If the feedback voltage ripple is so small
that the g_m amplifier and error comparator can’t sense it,
then the MIC2176 will lose control and the output voltage
is not regulated. In order to have some amount of V_FB
ripple, a ripple injection method is applied for low output
voltage ripple applications.
The applications are divided into three situations
according to the amount of the feedback voltage ripple:
1. Enough ripple at the feedback voltage due to the
large ESR of the output capacitors.
As shown in Figure 6a, the converter is stable
without any ripple injection. The feedback voltage
ripple is:
ΔV_FB(pp)
=
R2
R1+ R2
×
ESR
C_OUT
× ΔI_{L (pp)}
(29)
Figure 5. Voltage-Divider Configuration
The output voltage is determined by the equation:
V_OUT
=
V_FB
× (1+
R1)
R2
(27)
where ΔI_L(pp) is the peak-to-peak value of the
inductor current ripple.
2. Inadequate ripple at the feedback voltage due to the
small ESR of the output capacitors.
The output voltage ripple is fed into the FB pin
through a feedforward capacitor C_ff in this situation,
as shown in Figure 6b. The typical C_ff value is
between 1nF and 100nF. With the feedforward
capacitor, the feedback voltage ripple is very close
to the output voltage ripple:
ΔV_FB(pp) ≈ ESR × ΔI_{L (pp)}
(30)
3. Virtually no ripple at the FB pin voltage due to the
very-low ESR of the output capacitors:
where, V_FB = 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 in value, it will decrease the efficiency of the power
supply, especially at light loads. Once R1 is selected, R2
can be calculated using:
R2 = ^{VFB × R1}
V_OUT − V_FB
(28)
Ripple Injection
The V_FB ripple required for proper operation of the
MIC2176 g_m amplifier and error comparator is 20mV to
100mV. However, the output voltage ripple is generally
designed as 1% to 2% of the output voltage. For a low
output voltage, such as a 1V, the output voltage ripple is
only 10mV to 20mV, and the feedback voltage ripple is
November 2010
20
Figure 6a. Enough Ripple at FB
M9999-111710-A