MIC45116 Datasheet, PDF (25/42 Pages) Microchip Technology – 20V/6A DC/DC Power Module

• 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 feed-forward capacitor, C_FF in this
situation, as shown in Figure 5-3. The typical C_FF
value is between 1 nF and 100 nF.
EQUATION 5-11:
V_{FBPP} = ESR_COUT  I_{LPP}
With the feed-forward capacitor, the feedback
voltage ripple is very close to the output voltage
ripple.
V_OUT
MIC45116
FB
PGND
R_FB1
C_FF
C_OUT
R_FB2
ESR
FIGURE 5-3:
Inadequate Ripple at FB.
• Virtually no ripple at the FB pin voltage due to the
very low ESR of the output capacitors, such is the
case with ceramic output capacitors.
In this situation, the V_FB ripple waveform needs to be
generated by injecting suitable signal. A series RC
network between the SW pin and FB pin, R_INJ and
C_INJ as shown in Figure 5-4 injects a square-wave
current waveform into the FB pin, which, by means of
integration across the capacitor (C_FF), generates an
appropriate sawtooth FB ripple waveform.
VOUT
MIC45116
R_FB1
C_FF
C_OUT
FB
PGND
SW
C_INJ
R_INJ
R_FB2
ESR
FIGURE 5-4:
Circuit at FB.
External Ripple Injection
MIC45116
The injected ripple is calculated via:
EQUATION 5-12:
V _{F B  P P }
=
V_IN

K_div

D

1
–
D

--------1--------
f_SW  
Where:
V_IN
Power stage input voltage
D
Duty cycle
f_SW
Switching frequency
τ
(R_FB1//R_FB2//R_INJ) x C_FF
EQUATION 5-13:
Where:
K_div = _R----_I--_N---^R-_J---^F_+---^B--_R--¹--_F^/--/-_B-^R---₁-^F-/--/^B--_R-²---_F---_B---₂-
R_INJ
20 kΩ
In Equation 5-13 and Equation 5-14, it is assumed that
the time constant associated with C_FF must be much
greater than the switching period:
EQUATION 5-14:
--------1-------- = ^T-- « 1
f_SW   
If the voltage divider resistors R_FB1 and R_FB2 are in the
kΩ range, a C_FF of 1 nF to 100 nF can easily satisfy the
large time constant requirements.
5.5 Thermal Measurements and Safe
Operating Area (SOA)
Measuring the IC’s case temperature is recommended
to ensure it is within its operating limits. Although this
might seem like a very elementary task, it is easy to get
erroneous results. The most common mistake is to use
the standard thermal couple that comes with a thermal
meter. This thermal couple wire gauge is large, typically
22 gauge, and behaves like a heatsink, resulting in a
lower case measurement.
Two methods of temperature measurement are using a
smaller thermal couple wire or an infrared
thermometer. If a thermal couple wire is used, it must
be constructed of 36-gauge wire or higher (smaller wire
size) to minimize the wire heat-sinking effect. In
addition, the thermal couple tip must be covered in
either thermal grease or thermal glue to make sure that
the thermal couple junction is making good contact with
the case of the IC. Omega brand thermal couple
(5SC-TT-K-36-36) is adequate for most applications.
 2016 Microchip Technology Inc.
DS20005571A-page 25