178020601 Datasheet, PDF (14/29 Pages) Wurth Elektronik GmbH & Co. KG, Germany. – MagI³C Power Module VDRM - Variable Step Down Regulator Module

WPMDH1200601 / 171020601
MagI³C Power Module
VDRM - Variable Step Down Regulator Module
DESIGN FLOW
voltage rail rises at power-up. Most systems will benefit by using the precision Enable threshold to establish a system
under voltage lockout. The recommended approach is to choose an input UVLO level that is higher than the target
regulated output voltage for the stage. Without an Enable divider, this series of devices will attempt to turn on around
3.5 Vin. This would not be useful for a stage that ultimately might be creating 5Vout. Operation of the module on input
voltage conditions below the nominal output should be avoided. Systems that don't implement the Enable divider will
turn in early during the rise of Vin and might not have monotonic rise in output voltage. Many systems need smooth
rise in supply voltage. In the case of sequencing supplies, the divider is connected to a rail that becomes active
earlier in the power-up cycle than the MagI³C power module output rail. The two resistors should be chosen based on
the following ratio:
𝑅_{𝐸𝑁𝑇}
𝑅_{𝐸𝑁𝐵}
=
𝑉_{𝑈𝑉𝐿𝑂 (𝐸𝑋𝑇𝐸𝑅𝑁)}
1.18𝑉
–
1
(16)
VUVLO (EXTERN) = User programmable voltage threshold to turn the module ON/OFF.
The EN pin is internally pulled up to VIN and can be left floating for always-on operation. However, it is good practice
to use the enable divider and turn on the regulator when VIN is close to reaching its nominal value. This will guarantee
smooth startup and will prevent overloading the input supply.
Determine power losses and thermal requirements of the board
For example:
V_IN = 24𝑉, V_OUT = 3.3𝑉, I_OUT = 2𝐴, T_AMB(MAX) = 85°C and T_J(MAX) = 125°C
TAMB(MAX) is the maximum air temperature surrounding the module.
TJ(MAX) is the maximum value of the junction temperature according to the “OPERATING CONDITIONS” limit.
The goal of the calculation is to determine the characteristics of the required heat sink. In case of a surface mounted
module this would be the PCB (number of layers, copper area and thickness). These characteristics are reflected in
the value of the thermal resistance case to ambient: ƟCA.
The basic formula for calculating the operating junction temperature TJ of a semiconductor device is as follows:
T_J = 𝑃_{í µí°¼í µí°¶âˆ’í µí°¿í µí±œí µí± í µí±}  ∗ θ_JA + T_AMB
(17)
PIC-LOSS are the total power losses within the module IC and are related to the operating conditions.
ƟJA is the thermal resistance junction to ambient and calculated as:
θ_JA = θ_JC + θ_CA
(18)
ƟJC is the thermal resistance junction to case.
Combining equation (17) and (18) results in the maximum case-to-ambient thermal resistance:
θ_CA(MAX)
<
T_J−MAX − T_AMB(MAX)
𝑃_{í µí°¼í µí°¶âˆ’í µí°¿í µí±œí µí± í µí±} 
−
θ_JC
(19)
From section ”THERMAL SPECIFICATIONS“ the typical thermal resistance from junction to case (ƟJC) is defined
as 1.9 °C/W. Use the 85°C power dissipation curves in the “TYPICAL PERFORMANCE CURVES“ section to
estimate the PIC-LOSS for the application being designed.
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Würth Elektronik eiSos GmbH & Co. KG - Data Sheet - REV 1.0
14/29