MIC24053 Datasheet, PDF (23/32 Pages) Micrel Semiconductor – 12V, 9A High-Efficiency Buck Regulator

The process of sizing the ripple injection resistor and
capacitors is:
1. Select C_ff to feed all output ripples into the feed-
back pin and make sure the large time constant
assumption is satisfied. Typical choice of C_ff is
1 nF to 100 nF if R1 and R2 are in the kΩ range.
2. Select R_inj according to the expected feedback
voltage ripple using Equation 5-19:
EQUATION 5-19:
L_div
=
_----V_{-----F---B------p--p---}
V _IN

-_D-----^f-_--^S--_-^W--1----^_–-----^_D-----_
Then the value of R_inj is calculated as:
EQUATION 5-20:
R_inj
=

R1//
R2




-----1-----
K_div
–
1_
3. Select C_inj as 100 nF, which could be consid-
ered as short for a wide range of the frequen-
cies.
5.5 Setting Output Voltage
The MIC24053 requires two resistors to set the output
voltage as shown in Figure 5-4.
The output voltage is determined by Equation 5-21:
EQUATION 5-21:
V _OUT
=
V
FB



1
+
-^RR----12-_
Where:
V_FB
0.8V
A typical value of R1 can be between 3 kΩ and 10 kΩ.
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. Once R1 is selected, R2 can be
calculated using Equation 5-22
EQUATION 5-22:
R2 = -_V---^V-_O----_UF---^B-_T----^_–-----^RV----1_F---_B--
 2015 Microchip Technology Inc.
MIC24053
g_m Amp
V_REF
R1
FB
R2
FIGURE 5-4:
Configuration.
Voltage-Divider
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 MIC24053, as shown
in Figure 5-5. The inverting input voltage (V_INJ) is
clamped to 1.2V. As V_OUT increases, the swing of V_INJ
is clamped. The clamped V_INJ reduces the line
regulation because it is reflected as a DC error on the
FB terminal. Therefore, the maximum output voltage of
the MIC24053 should be limited to 5.5V to avoid this
problem.
SW
INTERNAL
RIPPLE
INJECTION
COMP V_INJ
COMPENSATION
g_m EA
FB
V_REF
0.8V
FIGURE 5-5:
Internal Ripple Injection.
5.6 Thermal Measurements
It is a good idea to measure the IC’s case temperature
to make sure it is within its operating limits. Although
this might seem like a very elementary task, it is easy
to get false 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 heat-sink, resulting in a
lower case measurement.
Two methods of temperature measurement are to use
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
DS20005668A-page 23