Micrel, Inc.
MIC28510 4A Evaluation Board
Features
March 2012
2
M9999-030712
or (408) 955-1690
MIC28510 evaluation
board, which is preset to 3.3V, is determined by the
feedback divider, shown in Equation 1:
Feedback Resistors
The output voltage on the
⎟⎟
⎠
⎜⎜
⎝
+
×
=
BOTTOM
REF
OUT
R
R4
1
V
V
where V
REF
= 0.8V, and R
BOTTOM
is one o
which corresponds
⎞
⎛
Eq.
1
f R5-R12,
to 0.9V, 1.0V, 1.2V, 1.5V, 1.8V,
2.5V, 3.3V, or 5V. Leaving the R
BOTTOM
open gives a
0.8V output voltage. All
above can be set by modifying R
BOTTOM
alu
according to Equation 2:
other voltages not listed
v
e
REF
OUT
REF
BOTTOM
V
V
R
−
=
Note that the output voltage should not be
V
R4
×
Eq.
2
5V due to the 6.3V voltage rating o
mmended
hown in the MIC28510 12V/4A
e output
d for 12V
ode
ng the
t critical
nt limit
om FET.
t of
easurement
The resistor, R13, is placed in series with the reg
feedback path. The control loop gain can be
measured by connecting an impedance analyzer
across the resistor and selecting the resistor value in
between 20
Ω
to 50
Ω
.
Setting the Switching Frequency
The MIC28510 is an adjustable-frequency,
synchronous buck regulator featuring a unique
digitally modified adaptive on-time control
architecture.
be adjusted between
ng the resistor divider
connected network consisting of R18 and R19 from
PVIN to PGND.
The switching frequency can
100kHz and 500kHz by changi
Figure 1. Switching Frequency Adjustment
switching frequency:
Equation 3 illustrates the estimated
set to
n the
exceed
R19
R18
R19
f
f
O
SW_ADJ
+
×
=
output capacitors.
If higher than 5V output is desired, it is reco
to use the designs s
Eq.
3
where f
O
= Switching Frequency when R18 is 100k
and R19 being open, f
O
should be typically 450kHz.
For more precise setting, it is recommended to use
Figure 2:
output evaluation board schematic, where th
capacitors, L1, R3, and R17 are optimize
output.
SW N
Test point J11 (V
SW
) is placed for monitori
switching waveform, one of the mos
waveforms for the converter.
Current Limit
The MIC28510 has internal FETs, and the curre
is implemented by sensing the R
DS-ON
of bott
The MIC28510 has a fixed current limi
(typical).
Loop Gain M
6.5A
ulator
Figure 2. Switching Frequency vs. R19
The evaluation board design is optimized for a
switching frequency of 250kHz. If the switching
frequency is programmed to either lower end or higher
end, the design needs optimization.
