TPS40040_15 Datasheet, PDF (16/38 Pages) Texas Instruments – LOW PIN COUNT, LOW VIN (2.5 V TO 5.5 V) SYNCHRONOUS BUCK DC-TO-DC CONTROLLER WITH ENABLE

TPS40040, TPS40041
SLUS700D – MARCH 2006 – REVISED DECEMBER 2007
www.ti.com
VDD Filter Resistor
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.
Thermal Shutdown
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.
Package Power Dissipation
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, Q_G, of the external MOSFETs, and the operating
frequency of the converter. Driver power, neglecting external gate resistance, is calculated from:
P_D(driver) + Q_G V_DRIVE F_SW Wńdriver
(1)
And the total power dissipation, assuming the same MOSFET is selected for both the high side and synchronous
rectifier is:
ǒ Ǔ P_T +
2 P_D
V_DRIVE
)
I
Q
V_DD W
(2)
or
P_T + ^ǒ2
G_Q
F
SW
)
I
Ǔ
Q
V_DD W
(3)
where I_Q 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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