Datasheet

10
20 30
40
2500
2000
1500
1000
500
0
f -SwitchingFrequency-kHz
s
V -InputVoltage-V
I
Skip
Shift
V =3.3V
O
( )
( )
L dc d
OUT sc
DIV
SWshift
ON IN L d
DS on
I R V V
t V I R V
f
f
æ ö
´ + +
ç ÷
= ´
ç ÷
- ´ +
è ø
( )
( )
L dc OUT d
SW max skip
ON IN L d
DS on
I R V V
1
t V I R V
f
æ ö
´ + +
ç ÷
= ´
ç ÷
- ´ +
è ø
TPS54140
www.ti.com SLVS889B –OCTOBER 2008–REVISED SEPTEMBER 2013
DETAILED DESCRIPTION (continued)
Selecting the Switching Frequency
The switching frequency that is selected should be the lower value of the two equations, Equation 12 and
Equation 13. Equation 12 is the maximum switching frequency limitation set by the minimum controllable on time.
Setting the switching frequency above this value will cause the regulator to skip switching pulses.
Equation 13 is the maximum switching frequency limit set by the frequency shift protection. To have adequate
output short circuit protection at high input voltages, the switching frequency should be set to be less than the
f
sw(maxshift)
frequency. In Equation 13, to calculate the maximum switching frequency one must take into account
that the output voltage decreases from the nominal voltage to 0 volts, the f
div
integer increases from 1 to 8
corresponding to the frequency shift.
In Figure 41, the solid line illustrates a typical safe operating area regarding frequency shift and assumes the
output voltage is zero volts, and the resistance of the inductor is 0.1Ω, FET on resistance of 0.2Ω and the diode
voltage drop is 0.5V. The dashed line is the maximum switching frequency to avoid pulse skipping. Enter these
equations in a spreadsheet or other software or use the SwitcherPro design software to determine the switching
frequency.
(12)
(13)
I
L
inductor current
Rdc inductor resistance
V
IN
maximum input voltage
V
OUT
output voltage
V
OUTSC
output voltage during short
Vc diode voltage drop
R
DS(on)
switch on resistance
t
ON(min)
minimum controllable on time
ƒ
DIV
frequency divide equals (1, 2, 4, or 8)
Figure 41. Maximum Switching Frequency vs. Input Voltage
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