SLOSE50A April 2020 – June 2021
PRODUCTION DATA
The output current and power dissipation capabilities of the device are heavily dependent on the PCB design and external system conditions. This section provides some guidelines for calculating these values.
Total power dissipation (PTOT) for the device is composed of three main components. These are the power MOSFET RDS(ON) (conduction) losses, the power MOSFET switching losses and the quiescent supply current dissipation. While other factors may contribute additional power losses, these other items are typically insignificant compared to the three main items.
PTOT = PCOND + PSW + PQ
PCOND can be calculated from the device RDS(ON) and average output current (IRMS).
PCOND = (IRMS)2 x (RDS(ONH) + RDS(ONL))
It should be noted that RDS(ON) has a strong correlation with the device temperature. A curve showing the normalized RDS(ON) with temperature can be found in the Typical Characteristics curves.
PCOND = (3.5-A)2 x (0.082-Ω + 0.082-Ω) = 2.009-W
PSW can be calculated from the nominal supply voltage (VM), average output current (IRMS), switching frequency (fPWM) and the device output rise (tRISE) and fall (tFALL) time specifications.
PSW = PSW_RISE + PSW_FALL
PSW_RISE = 0.5 x VM x IRMS x tRISE x fPWM
PSW_FALL = 0.5 x VM x IRMS x tFALL x fPWM
PSW_RISE = 0.5 x 24 V x 3.5 A x 100 ns x 35 kHz = 0.147 W
PSW_FALL = 0.5 x 24 V x 3.5 A x 100 ns x 35 kHz = 0.147 W
PSW = 0.147 W + 0.147 W = 0.294 W
PQ can be calculated from the nominal supply voltage (VM) and the IVM current specification.
PQ = VM x IVM = 24 V x 4 mA = 0.096 W
The total power dissipation (PTOT) is calculated as the sum of conduction loss, switching loss and the quiescent power loss.
PTOT = PCOND + PSW + PQ = 2.009-W + 0.294-W + 0.096-W = 2.399-W
For an ambient temperature of TA and total power dissipation (PTOT), the junction temperature (TJ) is calculated as
TJ = TA + (PTOT x RθJA)
Considering a JEDEC standard 4-layer PCB, the junction-to-ambient thermal resistance (RθJA) is 29.7 °C/W for the HTSSOP package and 39 °C/W for the VQFN package.
Assuming 25°C ambient temperature, the junction temperature for the HTSSOP package is calculated as -
TJ = 25°C + (2.399-W x 29.7 °C/W) = 96.25 °C
The junction temperature for the VQFN package is calculated as -
TJ = 25°C + (2.399-W x 39 °C/W) = 118.56 °C
It should be ensured that the device junction temperature is within the specified operating region.