WPMDH1100601 / 171010601
MagI3C Power Module
VDRM – Variable Step Down Regulator Module
Step 1 Set the output voltage (VOUT
)
The output voltage is determined by a divider of two resistors connected between VOUT 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:
RFBT
RFBB
VOUT
VFB
= (
) -1
(1)
These resistors should be chosen from values in the range of 1kΩ to 20kΩ.
A table of values for RFBT, RFBB, and RON is included in the “TYPICAL SCHEMATIC” section (page 38).
Step 2 Set the operating frequency (fSW) with RON
Many designs begin with a desired switching frequency in mind. For that purpose the following equation can be used:
VOUT
RON
≊
(2)
ꢀk ∙ fSW CCMꢂꢃ
ꢁ
where k = 1.3·10-10 C and fSW(CCM) is the switching frequency when the device is working in CCM (Continuous Conduction
Mode). While selecting the RON and the fSW(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 RON and the input voltage VIN. It is
calculated as follows:
(k ∙ RON
)
tON
=
(3)
VIN
The inverse relationship of tON and VIN gives a nearly constant switching frequency as VIN is varied. The TON is internally
limited to a minimum value of 150ns. Therefore RON should be selected such that the on-time at maximum VIN is greater than
150ns, as the following formula describes:
VIN MAXꢂ∙ t
ꢁ
ON-MIN
RON
≥
(4)
(5)
k
This limits the maximum operating frequency, which is governed by the following equation:
VOUT
fSW(MAX)
=
(VIN MAXꢂ∙t
)
ꢁ
ON-MIN
If the RON calculated in equation (2) is less than the minimum value determined in equation (4) a lower frequency should be
selected. Alternatively, VIN(MAX) can also be limited in order to keep the frequency unchanged.
we-online.com
Würth Elektronik eiSos GmbH & Co. KG – Data Sheet Rev. 1.0
© September 2017
18/54
WURTH [ WURTH ELEKTRONIK GMBH & CO. KG, GERMANY. ]
