Data Sheet

WPMDH1152401 / 171012402
MagI
3
C Power Module
VDRM – Variable Step Down Regulator Module
we-online.com Würth Elektronik eiSos GmbH & Co. KG – Data Sheet Rev. 2.0
© December 2017
18/54
Step 1 Set the output voltage (V
OUT
)
The output voltage is determined by a divider of two resistors connected between V
OUT
and ground. The midpoint of the
divider is connected to the FB input. The voltage at FB is compared to a 0.8V internal reference. In normal operation, an on-
time cycle is initiated when the voltage on the FB pin falls below 0.8V. The high-side MOSFET on-time cycle causes the
output voltage to rise and the voltage at the FB to exceed 0.8V. As long as the voltage at FB is above 0.8V, on time cycles
will not occur.
The ratio of the feedback resistors for the desired output voltage is:
R
FBT
R
FBB
=
V
OUT
V
FB
-1
(1)
The value of these resistors should be lower than 50kΩ.
A table of values for R
FBT
, R
FBB
, and R
ON
is included in the “TYPICAL SCHEMATIC” section (page 38).
Step 2 Set the operating frequency (f
SW
) with R
ON
Many designs begin with a desired switching frequency in mind. For that purpose the following equation can be used:
R
ON

V
OUT
kf
SW

CCM


(2)
where k = 1.3·10
-10
As and f
SW(CCM)
is the switching frequency when the device is working in CCM (Continuous Conduction
Mode). While selecting the R
ON
and the f
SW(CCM)
, the limitations in terms of minimum on-time and off-time must be taken into
account. The on-time of the MagI³C power module timer is determined by the resistor R
ON
and the input voltage V
IN
. It is
calculated as follows:
t
ON
=
(k R
ON
)
V
IN
(3)
The inverse relationship of t
ON
and V
IN
gives a nearly constant switching frequency as V
IN
is varied. The T
ON
is internally
limited to a minimum value of 150ns. Therefore R
ON
should be selected such that the on-time at maximum V
IN
is greater than
150ns, as the following formula describes:
R
ON
≥
V
IN

MAX


t
ON-MIN
k
(4)
This limits the maximum operating frequency, which is governed by the following equation:
f
SW(MAX)
=
V
OUT
(V
IN

MAX

t
ON-MIN
)
(5)
If the R
ON
calculated in equation (2) is less than the minimum value determined in equation (4) a lower frequency should be
selected. Alternatively, V
IN(MAX)
can also be limited in order to keep the frequency unchanged.