Texas Instruments TPS40051 manual Output Capacitor Selection, Mosfet selection

SLUU161 – April, 2003

4.6Output 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, as given in equation (7).

COUT(min)	+	IRIPPLE	+	3 A		+ 83 mF	(7)
		8 f VRIPPLE		8 300 kHz	15 mV

In this design, COUT(min) is 83 µ F with VRIPPLE=15 mV to allow for some margin. However, this only affects the capacitive component of the ripple voltage, and the final value of capacitance is generally influenced by ESR

and transient considerations. The voltage component due to the capacitor ESR.

CESR v	VRIPPLE	+	15 mA	+ 5 mW	(8)
	IRIPPLE		3 A

An additional consideration in the selection of the output inductor and capacitance value can be derived from examining the transient voltage overshoot which can be initiated with a load step from full load to no load. By equating the inductive energy with the capacitive energy the equation (9) can be derived:

2

L

￿￿IOH￿2 * ￿IOL￿2￿

m

(15 A)

(9)

CO v L V2I

1.7 H

+

+

+ 1034 mF

￿Vf￿

2

* ￿Vi￿

2

￿(1.9 V)

2

2

￿

* (1.8 V)

where

•

•

•

•

IOH = full load current

IOL = no load current

Vf = allowed transient voltage rise

Vi = initial voltage

For compactness while maintaining transient response capability, two 470-µ F POSCAP capacitors (C16, C17) are fitted in parallel. The total ESR of these capacitors is approximately 5 mΩ . An additional 47-µ F, 6.3-V ceramic capacitor C15 is placed in parallel with the POSCAPs to help suppress high frequency noise generated by the fast current transitions as the current switches between the input and output circuits during each switching cycle.

4.7MOSFET selection

Proper MOSFET selection is essential to optimize circuit efficiency. To operate with high current it is important to choose a package which allows the generated heat to be removed from the package as easily as possible. Various MOSFETs with a package similar to the SO–8 footprint are considered for this application, and devices with reduced junction-case thermal impedance are selected.

For the upper switch Q1, a Hitachi HAT2168H MOSFET with low gate charge (typically 27 nC at 10 V) and with

an RDS(on) of 6 mΩ is selected to keep the switching losses to a minimum. The low-side rectifier switch Q2 was chosen as a Hitachi HAT2167H, which has slightly more gate charge (43 nC at 10 V) but lower RDS(on) = 4.2 mΩ to minimize conduction losses. A schottky diode, D2, is placed across Q2 in this high current design to carry

some of the high circulating current during short circuit conditions.

8TPS40051-Based Design Converts 12-V Bus to 1.8 V at 15 A (SLUP195)