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(

)

(

)

(

)

Omin

IN max

Omin

O min

= 0.118 ×

× Rin

+ V

× R

(31)

POWER DISSIPATION ESTIMATE

TPS54331

SLVS839 – JULY 2008 ....................................................................................................................................................................................................... www.ti.com

The equation assumes maximum on resistance for the internal high-side FET.

The lower limit is constrained by the minimum controllable on time which may be as high as 160 ns. The

approximate minimum output voltage for a given input voltage and minimum load current is given by Equation 31:

Where:

VIN max = Maximum input voltage

IO min = Minimum load current

VD = Catch diode forward voltage

RL = Output inductor series resistance

This equation assumes nominal on-resistance for the high-side FET and accounts for worst case variation of

operating frequency set point. Any design operating near the operational limits of the device should be carefully

checked to assure proper functionality.

The following formulas show how to estimate the device power dissipation under continuous conduction mode

operations. They should not be used if the device is working in the discontinuous conduction mode (DCM) or

pulse skipping Eco-modeTM.

The device power dissipation includes:

1) Conduction loss: Pcon = IOUT2 x Rds(on) x VOUT/VIN

2) Switching loss: Psw = 0.5 x 10-9 x VIN2 x IOUT x Fsw

3) Gate charge loss: Pgc = 22.8 x 10-9 x Fsw

4) Quiescent current loss: Pq = 0.11 x 10-3 x VIN

Where:

IOUT is the output current (A).

Rds(on) is the on-resistance of the high-side MOSFET (

Ω).

VOUT is the output voltage (V).

VIN is the input voltage (V).

Fsw is the switching frequency (Hz).

Ptot = Pcon + Psw + Pgc + Pq

For given TA , TJ = TA + Rth x Ptot.

For given TJMAX = 150°C, TAMAX = TJMAX– Rth x Ptot.

Where:

Ptot is the total device power dissipation (W).

TA is the ambient temperature (°C).

TJ is the junction temperature (°C) .

Rth is the thermal resistance of the package (°C/W).

TJMAX is maximum junction temperature (°C).

TAMAX is maximum ambient temperature (°C).

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