TPS57140-Q1 Datasheet, PDF (21/49 Pages) Texas Instruments – 1.5-A 42-V STEP-DOWN SWIFT™ DC/DC CONVERTER WITH Eco-mode™ CONTROL

TPS57140-Q1
www.ti.com
SLVSAP3A – DECEMBER 2010 – REVISED MARCH 2011
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 42, 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.
f_{SW(max skip)}
=
1
t_ON
æ
´ç
ç
è
I_L ´ R_dc + V_OUT + V_d
V_IN - I_L ´ R_DS(on) + V_d
ö
÷
÷
ø
(12)
f_SWshift
=
f_DIV
t_ON
æ I_L
´ç
´ R_dc
+
V_{OUT(sc )}
+
V_d
ç
è
V_IN - I_L ´ R_DS(on) + V_d
ö
÷
÷
ø
(13)
I_L
Rdc
V_IN
V_OUT
V_OUTSC
Vd
R_DS(on)
t_ON
ƒ_DIV
inductor current
inductor resistance
maximum input voltage
output voltage
output voltage during short
diode voltage drop
switch on resistance
controllable on time
frequency divide equals (1, 2, 4, or 8)
2500
2000
1500
Skip
1000
V_O = 3.3 V
Shift
500
0
10
20
30
40
V_I - Input Voltage - V
Figure 42. Maximum Switching Frequency vs. Input Voltage
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