MIC28304 Datasheet, PDF (29/39 Pages) Micrel Semiconductor – 70V 3A Power Module

Micrel, Inc.
The feedback voltage ripple is:
ΔV_FB(PP)
=
R11
R1 + R11
×
ESR
C_OUT
× ΔI_L(PP)
Eq. 16
Where:
ΔI_L(PP) = 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, such is the case
with ceramic output capacitor.
The output voltage ripple is fed into the FB pin
through a feed-forward capacitor C_ff in this situation,
as shown in Figure 11. The typical C_ff value is
between 1nF and 100nF.
Figure 11. Invisible Ripple at FB
With the feed-forward capacitor, the feedback voltage
ripple is very close to the output voltage ripple:
ΔV_FB(PP) ≈ ESR × ΔI_L(PP)
Eq. 17
3. Virtually no ripple at the FB pin voltage due to the
very-low ESR of the output capacitors.
In this situation, the output voltage ripple is less than
20mV. Therefore, additional ripple is injected into the
FB pin from the switching node SW via a resistor R_inj
and a capacitor C_inj, as shown in Figure 11. The
injected ripple is:
ΔV_FB(pp)
=
V_IN
× K _div
× D × (1- D) ×
1
f_SW ×τ
Eq. 18
K _div
=
R1//R11
R _inj + R1//R11
Eq. 19
V_IN = Power stage input voltage
D = Duty cycle
f_SW = Switching frequency
τ = (R1//R11//R_inj) × C_ff
MIC28304
In Equations 18 and 19, 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
Eq. 20
If the voltage divider resistors R1 and R11 are in the kΩ
range, then a C_ff of 1nF to 100nF can easily satisfy the
large time constant requirements. 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 R11 are in kΩ range.
Step 2. Select R_inj according to the expected feedback
voltage ripple using Equation 22:
K _div
=
ΔV_FB(pp)
V_IN
×
f_SW ×τ
D × (1− D)
Eq. 21
Then the value of R_inj is obtained as:
R _inj
= (R1//R11) × ( ¹
K _div
− 1)
Eq. 22
Step 3. Select C_inj as 100nF, which could be considered
as short for a wide range of the frequencies.
Table 3 summarizes the typical value of components for
particular input and output voltage, and 600kHz
switching frequency design, for details refer to the Bill of
Materials section.
Where:
March 25, 2014
29
Revision 1.1