MIC2174_10 Datasheet, PDF (19/27 Pages) Micrel Semiconductor – Synchronous Buck Controller Featuring Adaptive On-Time Control 40V Input, 300kHz

Micrel, Inc.
2. Inadequate ripple at the FB 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 7b. The typical C_ff value is
between 1nF and 100nF. With the feedforward
capacitor, the FB voltage ripple is very close to the
output voltage ripple:
ΔV_FB(pp) ≈ ESR × ΔI_{L (pp)}
(36)
3. Virtually no ripple at the FB pin voltage is due to the
very low ESR of the output capacitors.
Figure 7a. Enough Ripple at FB
Figure 7b. Inadequate Ripple at FB
Figure 7c. Invisible Ripple at FB
MIC2174/MIC2174C
In this situation, the output voltage ripple is less than
20mV. Therefore, additional ripple is injected into the FB
pin from the switching node LX via a resistor R_inj and a
capacitor C_inj, as shown in Figure 7c. The injected ripple
is:
1
ΔV_FB(pp) = V_HSD ×K _div ×D×(1- D)× _{fSW × τ}
K _div
=
R1//R2
R_inj + R1//R2
(37)
(38)
where
V_HSD = Power stage input voltage at HSD pin
D = Duty Cycle
f_SW = switching frequency
τ = (R1//R2//R_inj) × C_ff
In the equations (37) and (38), it is assumed that the
time constant associated with C_ff must be much greater
than the switching period:
1 ₌ T _{<< 1}
f_SW ×τ τ
If the voltage divider resistors R1 and R2 are in the kΩ
range, a C_ff of 1nF to 100nF can easily satisfy the large
time constant consumption. Also, a 100nF injection
capacitor C_inj is used in order to be considered as short
for a wide range of the frequencies.
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_HSD
× ^{fSW × τ}
D × (1 − D)
(39)
Then the value of Rinj is obtained as:
R _inj
= (R1//R2) × ( ¹
K _div
− 1)
(40)
September 2010
19
M9999-091310-C