Texas Instruments TPS40003 Output Capacitor Selection, MOSFET Selection, Short Circuit Protection

SLUU130A – September 2002 – Revised February 2003

4.4Output Capacitor Selection

Selection of the output capacitor is based on many application variables, including function, cost, size, and availability. The minimum allowable output capacitance is determined by the amount of inductor ripple current and the allowable output ripple in equation (3).

COUT(min)	+		IRIPPLE	+	1.25 A		+ 22 mF	(3)
		8 f VRIPPLE			8 600 kHz	12 mV

In this design, COUT(min) is 22 ∝ F with VRIPPLE = 12 mV to allow for some margin. However, this only affects the capacitive component of the ripple voltage. In addition, the voltage component due to the capacitor ESR must

be considered in equation (4).

ESRCout v	VRIPPLE	+	0.012 V	+ 9.6 mW	(4)
	IRIPPLE		1.25 A

For compactness while maintaining transient response capability, two 22-∝F ceramic capacitors are fitted in parallel. The total ESR of these capacitors is below 3 mΩ , and contributes only a few mV to the output voltage ripple.

4.5MOSFET Selection

The small physical size of this design requires the use of a single SO-8 package which contains dual N-channel

MOSFETs. MOSFETs with an RDS(on) of 18 mΩ are selected to keep the conduction losses to a manageable amount at full load.

4.6Short Circuit Protection

The TPS40003 implements short circuit protection by comparing the voltage across the topside MOSFET while

it is on to a voltage dropped from VDD by RLIM due to an internal current source of 15 ∝ A inside pin 1. Due to tolerances in the current source and variations in the power MOSFET on-voltage versus temperature, the short

circuit level can protect against gross overcurrent conditions only, and should be set higher than rated load. In this particular case, RLIM is selected as:

2.5	IOUT 0.018 W		(5)
RLIM + R1 +		+ 15 kW
	15 mA

For this design, RLIM = 15 kΩ , and the factor of 2.5 in the equation accounts for the variations in component tolerances over temperature and output current ripple. The high currents that are switched under short circuit

conditions may cause SW pin 8 to be driven below ground several volts, possibly injecting substrate current which can cause improper operation of the device. A 3.3-Ωresistor has been placed in series with this pin to limit its excursion to safe levels.

TPS40003-Based 5-A Converter in Less Than One Square Inch 7