TPS57140-Q1_16 Datasheet, PDF (18/53 Pages) Texas Instruments – Step-Down DC-DC Converter

TPS57140-Q1
SLVSAP3D – DECEMBER 2010 – REVISED FEBRUARY 2016
www.ti.com
Feature Description (continued)
7.3.12 Selecting the Switching Frequency
The selected switching frequency should be the lower value of the two equations, Equation 8 and Equation 9.
Equation 8 is the maximum switching frequency limitation set by the minimum controllable on-time. Setting the
switching frequency above this value causes the regulator to skip switching pulses.
Equation 9 is the maximum switching-frequency limit set by the frequency-shift protection. To have adequate
output short-circuit protection at high input voltages, set the switching frequency to be less than the ƒ_SW(maxshift)
frequency. In Equation 9, to calculate the maximum switching frequency, take into account that the output
voltage decreases from the nominal voltage to 0 V and that the ƒ_DIV integer increases from 1 to 8, corresponding
to the frequency shift.
In Figure 33, the solid line illustrates a typical safe operating area regarding frequency shift and assumes the
output voltage is 0 V, the resistance of the inductor is 0.1 Ω, the FET on-resistance is 0.2 Ω, and the voltage
drop of the diode is 0.5 V. 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
çè
I_L ´ R_dc + V_OUT + V_d
V_IN - I_L ´ R_DS(on) + V_d
ö
÷
÷ø
(8)
f_SWshift
=
f_DIV
t_ON
´
æ
ç
çè
I_L ´ R_dc + V_OUT(sc) + V_d
V_IN - I_L ´ R_DS(on) + V_d
ö
÷
÷ø
where
• I_L = Inductor current
• Rdc = Inductor resistance
• V_IN = Maximum input voltage
• V_OUT = Output voltage
• V_OUTSC = Output voltage during short
• Vd = Diode voltage drop
• r_DS(on) = Switch on-resistance
• t_ON = Controllable on-time
• ƒ_DIV = Frequency divide (equals 1, 2, 4, or 8)
(9)
2500
V_O = 3.3 V
2000
Shift
1500
Skip
1000
500
0
10
20
30
40
V_I - Input Voltage - V
Figure 33. Maximum Switching Frequency vs Input Voltage
18
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