Datasheet

OUT
(diode)(RMS) OUT OUT
IN
V
I I (1 D) = I 1
V
æ ö
» - -
ç ÷
è ø
1.1
PP
1
V = I ES R + I ESR
8 COUT f
é ù
æ ö
D D ´ » D ´
ç ÷ê ú
´ ´
è ø
ë û
( )
2
O UT
OU T
IN OU T
L ΔI
C =
V V ΔV
´
- ´
TPS75003-EP
www.ti.com SGLS311A –DECEMBER 2006– REVISED MARCH 2011
Diode Selection (Buck Controllers)
The diode is off when the PMOS is on, and on when the PMOS is off. Since it will be turned on and off at a
relatively high frequency, a Schottky diode is recommended for good performance. The peak current rating of the
diode should exceed the peak current limit set by the sense resistor R
IS1,2
. A diode with low reverse leakage
current and low forward voltage at operating current will optimize efficiency. Equation 10 calculates the estimated
average power dissipation.
(10)
Output Capacitor Selection (Buck Controllers)
The output capacitor is selected based on output voltage ripple and transient response requirements. As a result
of the nature of the hysteretic control loop, a minimum ESR of a few tens of mΩ should be maintained for good
operation unless a feed-forward resistor is used. Low ESR bulk tantalum or PosCap capacitors work best in most
applications. A 1-μF ceramic capacitor can be used in parallel with this capacitor to filter higher frequency spikes.
The output voltage ripple can be estimated by Equation 11.
(11)
To calculate the capacitance needed to achieve a given voltage ripple as a result of a load transient from zero
output to full current, use Equation 12.
(12)
If only ceramic or other very low ESR output capacitor configurations are desired, additional voltage ripple must
be passed to the feedback pin. See Application Note, Using Ceramic Output Capacitors with the TPS6420x Buck
Controllers (SLVA210), for detailed application information.
Output Voltage Ripple Effect on V
OUT
(Buck Controllers)
Output voltage ripple causes V
OUT
to be higher or lower than the target value by half of the peak-to-peak voltage
ripple. For minimum on-time, the ripple adds to the voltage; for minimum off-time, it subtracts from the voltage.
Soft-Start Capacitor Selection (Buck Controllers)
BUCK1 is discussed in this section; it is identical to BUCK2. Soft-start is implemented on the buck controllers by
ramping current limit from 0 to its target value (set by R1) over a user-defined time. This time is set by the
external soft-start cap connected to pin SS1. If SS1 is left open, a small on-chip capacitor will provide a current
limit ramp time of approximately 250 μs. Figure 25 shows the effects of R1 and SS1 on the current limit start-up
ramp.
Copyright © 2006–2011, Texas Instruments Incorporated 17
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