MIC45208 Datasheet, PDF (17/31 Pages) Micrel Semiconductor – 26V 10A DC-to-DC Power Module

Micrel, Inc.
Application Information
Setting the Switching Frequency
The MIC45208 switching frequency can be adjusted by
changing the value of resistors R1 and R2.
Figure 6. Switching Frequency Adjustment
Equation 5 gives the estimated switching frequency:
f_SW
 f_O

R2
R1 R2
Eq. 5
MIC45208
The switching frequency also depends upon VIN, VOUT
and load conditions as MIC45208 uses and adaptive ON-
time architecture as explained in Theory of Operation.
At lower switching frequencies, the I_RMS current will
increase due to higher ripple current. Designs need to
take this into account when calculating for safe operating
area.
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, OS-CON and
POSCAP. The output capacitor’s ESR is usually the main
cause of the output ripple. The MIC45208 requires ripple
injection and the output capacitor ESR affects the control
loop from a stability point of view.
The maximum value of ESR is calculated as in Equation
6:
ESR_COUT

ΔV_OUT(PP)
ΔI_L(PP)
Eq. 6
Where:
f_O = 600kHz (typical per Electrical Characteristics⁽⁵⁾ table)
R1= 100kΩ is recommended.
R2 needs to be selected in order to set the required
switching frequency.
Where:
ΔV_OUT(PP) = Peak-to-peak output voltage ripple
ΔI_L(PP) = Peak-to-peak inductor current ripple
Switching Frequency
800
700 ^VOUT ^{= 5V}
600
VIN = 12V
500
400
300
200
R1 = 100kΩ
100
0
10.00
100.00
1000.00
R2 (kΩ)
10000.00
Figure 7. Switching Frequency vs. R2
May 29, 2014
17
Revision 1.2