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VDD Filter Resistor

Thermal Shutdown

Package Power Dissipation

P

D(driver) + QG

V

DRIVE

F

SW W driver

(1)

P

T +

2

P

D

V

DRIVE

) I

Q

V

DD W

(2)

P

T + 2

G

Q

F

SW ) IQ

V

DD W

(3)

TPS40040, TPS40041

SLUS700D – MARCH 2006 – REVISED DECEMBER 2007

To further limit the noise on VDD, a small 1-

Ω to 2-Ω resistor may be placed between the input voltage and the

VDD pin to create a small filter to VDD. The resistor should connect near the drain of the upper switch MOSFET

to prevent trace IR drops from increasing the sensed voltage drop. The resistor itself should be placed close to

Pin 4.

The current through the resistor includes the device's no-load switching current of 2 mA plus gate switching

current. The voltage drop induced across this resistor reduces the VDD-to-SW voltage sensed by the over

current protection circuitry within the device. This results with the apparent voltage drop across the upper switch

MOSFET being increased, thereby decreasing the current at which protection will occur. To minimize this effect,

the resistor value should be selected to yield less than a 25-mV drop.

If the junction temperature of the device reaches the thermal shutdown level, the PWM and the oscillator are

turned off and HDRV and LDRV are driven off. When the junction cools to the required level, the PWM soft starts

as during a normal power-up cycle.

The power dissipation in a controller is largely dependent on the MOSFET driver currents and the input voltage.

The driver current is proportional to the total gate charge, QG, of the external MOSFETs, and the operating

frequency of the converter. Driver power, neglecting external gate resistance, is calculated from:

And the total power dissipation, assuming the same MOSFET is selected for both the high side and synchronous

rectifier is:

or

where IQ is the quiescent operating current (neglecting drivers).

The max power capability of the PowerPad™ package is dependent on the layout as well as air flow. The

thermal impedance from junction-to-air assuming 2-oz copper trace and thermal pad with solder and no air flow

is detailed in Reference [5].

16

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