MIC28510_12 Datasheet, PDF (22/30 Pages) Micrel Semiconductor – 75V/4A Hyper Speed Control Synchronous DC/DC Buck Regulator

Micrel, Inc.
The output voltage is determined by Equation 24:
V_O

V_FB

1

R1
R2


Eq. 24
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, it will decrease the efficiency of the power supply,
especially at light loads. Once R1 is selected, R2 can be
calculated using:
R2

V_FB
V_OUT
 R1
 V_FB
Eq. 25
The Figure 9 shows the typical input output relationship.
The typical operating point should fall under the non-
grey part of the graph shown in Figure 9. This should be
used in conjunction with the recommended component
values indicated in Table 1 and Bill of Matrials Table.
Figure 9. Output Voltage vs Input Voltage
MIC28510
V_OUT (V) V_IN (V)
L
C_OUT
0.8V to
3.3V
5V to
75V
10H 100F
R3
(R_INJ)
16.5k
C3
(C_INJ)
0.1F
5V
5.88V to 10H 100F
10k 0.1F
75V
12V
15V to
27H 2x47F
45.3k 0.1F
75V
24V
30V to
47H 10F+220F 133k 0.1F
75V
Table 1. Recommended Component values
`
The inverting input voltage V_INJ is clamped to 1.2V. As
the injected ripple increases, the swing of V_INJ will be
clamped. The clamped V_INJ reduces the line regulation
because it is reflected back as a DC error on the FB
terminal.
Thermal Measurements
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.
Wherever possible, an infrared thermometer is
recommended. The measurement spot size of most
infrared thermometers is too large for an accurate
reading on a small form factor ICs. However, an IR
thermometer from Optris has a 1mm spot size, which
makes it a good choice for measuring the hottest point
on the case. An optional stand makes it easy to hold the
beam on the IC for long periods of time.
March 2012
22
M9999-030912-A