WPMDH1100601 / 171010601
MagI3C Power Module
VDRM – Variable Step Down Regulator Module
Step 5 Select the feed forward capacitor (CFF)
A feed-forward capacitor CFF is placed in parallel with RFBT that bypasses AC ripple directly to the feedback pin from the
output to support the internal ripple generator. This capacitor also affects the load step transient response. Its value is
usually determined experimentally by load stepping between DCM and CCM and adjusting for best transient response and
minimum output ripple. A value of 22nF has been practically evaluated as the best choice. The feed forward capacitor CFF
should be located close to the FB pin.
Step 6 Select soft-start capacitor (CSS)
A minimum soft-start capacitance of 22nF is required. Programmable soft-start permits the regulator to slowly ramp up to its
steady state operating point after being enabled, thereby reducing the input current at start-up and slowing the output
voltage rise-time to prevent overshoot. Upon turn-on, after all UVLO conditions have been passed, an internal 8µA current
source begins charging the external soft-start capacitor. The soft-start capacitor can be calculated with:
8μA
CSS= tSS
∙
(17)
0.8V
Where tSS in the desired soft-start time in milliseconds. The use of a 22nF capacitor results in 2.2ms soft-start duration.
As the soft-start input exceeds 0.8V the output of the power stage will be in regulation. The soft-start capacitor continues
charging until it reaches approximately 3.8V on the SS pin. Voltage levels between 0.8V and 3.8V do not influence the
regulation of the output voltage.
The picture below shows the output voltage under three different soft-start conditions:
Output Voltage at Start Up with Different Soft-Start Capacitors at VOUT = 5V
6,0
5,5
Enable
5,0
4,5
4,0
3,5
3,0
2,5
2,0
1,5
1,0
0,5
0,0
Css = 22nF
Css = 47nF
Css = 68nF
0
2
4
6
8
10
12
Time [ms]
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Würth Elektronik eiSos GmbH & Co. KG – Data Sheet Rev. 1.0
© September 2017
28/54
WURTH [ WURTH ELEKTRONIK GMBH & CO. KG, GERMANY. ]
