WPMDH1100601 / 171010601
MagI3C Power Module
VDRM – Variable Step Down Regulator Module
The second contributing factor is the voltage drop due to the discharge of the output capacitor. In order to estimate this
contribution, the behavior of the inductor current during the transient should be analyzed (see picture below, ESR
contribution neglected).
At the transition, the device tries to reach the new steady state as fast as possible by increasing the inductor current. This
can be achieved only by modulating the off-time tOFF since the on-time is fixed and defined by RON. The device has a
minimum tOFF (tOFF-MIN = 260ns typ.). Therefore, as long as the new steady state is not achieved, the inductor current
increases by performing consecutive cycles of tON and tOFF-MIN. During the transition to the new output current, the load
demand is supported by the energy stored in the output capacitor. For that reason, the output voltage drops until the
average inductor current reaches the new output current. The time for reaching this condition (td) can be calculated as
follows:
∆I
2
(∆IOUT
+
L )∙L∙(tON+tOFF-MIN
)
(14)
td=
VIN∙tON-VOUT∙(tON+tOFF-MIN
)
The td calculated above represents the worst case, i.e. it is supposed that the load transient occurs when the inductor
∆IL
current has its minimum value (IOUT
−
ꢂ.
2
IL, VOUT
200
3
150
100
50
2,5
IOUT2
LOAD
TRANSITION
2
tOFFmin
VOUT
0
1,5
1
∆VOUT
-50
IL
-100
-150
-200
IOUT1
0,5
tON
0
t
td
The selection of the COUT is related to the td as well as to the current step ∆IOUT and the max allowed voltage drop ∆VOUT, as
shown by the following equation:
∆I
2
(∆IOUT
+
L )∙td
(15)
COUT
≥
2∙∆VOUT
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Würth Elektronik eiSos GmbH & Co. KG – Data Sheet Rev. 1.0
© September 2017
24/54
WURTH [ WURTH ELEKTRONIK GMBH & CO. KG, GERMANY. ]
