TPS54260
www.ti.com
SLVSA86 –MARCH 2010
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
fsw(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 fdiv integer increases from 1 to
8 corresponding to the frequency shift.
In Figure 40, 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 0.130Ω, 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.
æ
ç
ç
è
ö
÷
÷
ø
I
´ Rdc + VOUT + Vd
æ
ç
è
ö
÷
ø
(
)
1
L
fSW maxskip
=
´
(
)
tON
VIN -IL ´ Rhs + Vd
(
)
(12)
(13)
æ
ç
ç
è
ö
÷
÷
ø
(IL ´Rdc + VOUTSC + Vd)
fdiv
fSW shift
=
´
(
)
tON
V -I x Rhs + Vd
IN L
(
)
IL
inductor current
Rdc
VIN
VOUT
inductor resistance
maximum input voltage
output voltage
VOUTSC
Vd
output voltage during short
diode voltage drop
switch on resistance
controllable on time
RDS(on)
tON
ƒDIV
frequency divide equals (1, 2, 4, or 8)
2500
VO = 3.3 V
2000
1500
Shift
Skip
1000
500
0
20
30
40
VI - Input Voltage - V
10
50
60
Figure 40. Maximum Switching Frequency vs. Input Voltage
Copyright © 2010, Texas Instruments Incorporated
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