MIC28304 Datasheet, PDF (26/39 Pages) Micrel Semiconductor – 70V 3A Power Module

Micrel, Inc.
Application Information
Simplified Input Transient Circuitry
The 76V absolute maximum rating of MIC28304 allows
simplifying the transient voltage suppressor on the input
supply side which is very common in industrial
applications. The input supply voltage V_IN Figure 5 may
be operating at 12V input rail most of the time, but can
encounter noise spike of 60V for a short duration. By
using MIC28304, which has 76V absolute maximum
voltage rating, the input transient suppressor is not
needed. Which saves on component count, form factor,
and ultimately the system becomes less expensive.
Figure 5. Simplified Input Transient Circuitry
Setting the Switching Frequency
The MIC28304 switching frequency can be adjusted by
changing the value of resistor R19. The top resistor of
100kΩ is internal to module and is connected between
VIN and FREQ pin, so the value of R19 sets the
switching frequency. The switching frequency also
depends upon VIN, V_OUT and load conditions.
Figure 6. Switching Frequency Adjustment
MIC28304
Equation 5 gives the estimated switching frequency:
f_{SW _ ADJ}
=
f_O
× ^R19
R19 + 100kΩ
Eq. 5
Where:
f_O = Switching frequency when R19 is open
For more precise setting, it is recommended to use
Figure 7:
Switching Frequency
800
700 ^VOUT ^{= 5V}
I_OUT = 2A
600
VIN = 12V
500
VIN =48V
400
300
200
100
0
10.00
100.00
1000.00
R19 (k Ohm)
10000.00
Figure 7. Switching Frequency vs. R19
Output Capacitor Selection
The type of the output capacitor is usually determined by
the application and its equivalent series resistance
(ESR). Voltage and RMS current capability are two other
important factors for selecting the output capacitor.
Recommended capacitor types are MLCC, tantalum, low-
ESR aluminum electrolytic, OS-CON and POSCAP. The
output capacitor’s ESR is usually the main cause of the
output ripple. The MIC28304 requires ripple injection and
the output capacitor ESR effects the control loop from a
stability point of view.
March 25, 2014
26
Revision 1.1