MIC28510EV Datasheet, PDF (2/14 Pages) Micrel Semiconductor – 75V/4A Hyper Speed Control™ Synchronous DC/DC Buck Regulator

Micrel, Inc.
Features
Feedback Resistors
The output voltage on the MIC28510 evaluation
board, which is preset to 3.3V, is determined by the
feedback divider, shown in Equation 1:
V_OUT
=
V_REF
×
⎜⎜⎝⎛¹⁺
R
R4
BOTTOM
⎟⎟⎠⎞
Eq. 1
where V_REF = 0.8V, and R_BOTTOM is one of R5-R12,
which corresponds 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 other voltages not listed
above can be set by modifying R_BOTTOM value
according to Equation 2:
R_BOTTOM
=
R4 × V_REF
V_OUT − V_REF
Eq. 2
Note that the output voltage should not be set to
exceed 5V due to the 6.3V voltage rating on the
output capacitors.
If higher than 5V output is desired, it is recommended
to use the designs shown in the MIC28510 12V/4A
output evaluation board schematic, where the output
capacitors, L1, R3, and R17 are optimized for 12V
output.
SW Node
Test point J11 (V_SW) is placed for monitoring the
switching waveform, one of the most critical
waveforms for the converter.
Current Limit
The MIC28510 has internal FETs, and the current limit
is implemented by sensing the R_DS-ON of bottom FET.
The MIC28510 has a fixed current limit of 6.5A
(typical).
Loop Gain Measurement
The resistor, R13, is placed in series with the regulator
feedback path. The control loop gain can be
measured by connecting an impedance analyzer
across the resistor and selecting the resistor value in
between 20Ω to 50Ω.
Setting the Switching Frequency
The MIC28510 is an adjustable-frequency,
synchronous buck regulator featuring a unique
digitally modified adaptive on-time control
architecture.
March 2012
MIC28510 4A Evaluation Board
The switching frequency can be adjusted between
100kHz and 500kHz by changing the resistor divider
connected network consisting of R18 and R19 from
PVIN to PGND.
Figure 1. Switching Frequency Adjustment
Equation 3 illustrates the estimated switching frequency:
f_{SW_ADJ}
= f_O
× ^R19
R18 + R19
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:
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.
2
M9999-030712
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