Datasheet

R
RT
+ K
PH
ǒ
39.2 10
3
f
*1.041
PH
* 7
Ǔ
F
RPL
+ N
PH
f
PH
R
T
- Timing Resistance - kΩ
SWITCHING FREQUENCY
vs
TIMING RESISTANCE
0
10000
100
50
100 150 200 250 300
f
SW
- Switching Frequency - kHz
TPS40090
TPS40091
SLUS578B –OCTOBER 2003– REVISED MAY 2006 www.ti.com
SETTING THE SWITCHING FREQUENCY
The clock frequency is programmed by the value of the timing resistor connected from the RT pin to ground.
(7)
where:
K
PH
is a coefficient that depends on the number of active phases. For two-phase and three-phase
configurations, K
PH
= 1.333. For four-phase configurations, K
PH
= 1. f
PH
is a single phase frequency, kHz. The
RT resistor value is returned by the last expression in kΩ.
To calculate the output ripple frequency, use the following equation:
where:
• N
PH
is a number of phases used in the converter. (8)
The switching frequency of the controller can be synchronized to an external clock applied to the EN/SYNC pin.
The external frequency should be somewhat higher than the free-running clock frequency for synchronization to
take place.
Figure 3.
12 Submit Documentation Feedback Copyright © 2003–2006, Texas Instruments Incorporated
Product Folder Link(s): TPS40090 TPS40091