Datasheet

T
1.0888
SW
206,003
R (kΩ)
f (kHz)
=
L dc OUT d
SW(maxskip)
ON IN L DS(on) d
I R V V
1
f
t V I r V
æ ö
æ ö
´ + +
= ´
ç ÷
ç ÷
ç ÷
- ´ +
è ø
è ø
L dc OUTSC d
DIV
SW(shift)
ON IN L DS(on) d
I R V V
f
f
t V I r V
æ ö
æ ö
´ + +
= ´
ç ÷
ç ÷
ç ÷
- ´ +
è ø
è ø
TPS5401
SLVSAB0 –DECEMBER 2010 www.ti.com
DETAILED DESCRIPTION (continued)
Constant Switching Frequency and Timing Resistor (RT/CLK Pin)
The switching frequency of the TPS5401 is adjustable over a wide range from approximately 100 kHz to 2500
kHz by placing a resistor on the RT/CLK pin. The RT/CLK pin voltage is typically 0.5 V and must have a resistor
to ground to set the switching frequency. To determine the timing resistance for a given switching frequency, use
Equation 9 or the curves in Figure 5 or Figure 6. To reduce the solution size one would typically set the switching
frequency as high as possible, but tradeoffs of the supply efficiency, maximum input voltage, and minimum
controllable on-time should be considered.
The minimum controllable on-time is typically 130 ns and limits the maximum operating input voltage.
The maximum switching frequency is also limited by the frequency-shift circuit. More discussion on the details of
the maximum switching frequency is located as follows.
(9)
Overcurrent Protection and Frequency Shift
The TPS5401 implements current-mode control, which uses the COMP pin voltage to turn off the high-side
MOSFET on a cycle-by-cycle basis. Each cycle, the switch current and COMP pin voltage are compared; when
the peak switch current intersects the COMP voltage, the high-side switch is turned off. During overcurrent
conditions that pull the output voltage low, the error amplifier responds by driving the COMP pin high, increasing
the switch current. The error amplifier output is clamped internally, which functions as a switch-current limit.
To increase the maximum operating switching frequency at high input voltages, the TPS5401 implements a
frequency shift. The switching frequency is divided by 8, 4, 2, and 1 as the voltage ramps from 0 to 0.8 volts on
the VSENSE pin.
The device implements a digital frequency shift to enable synchronizing to an external clock during normal
start-up and fault conditions. Because the device can only divide the switching frequency by 8, there is a
maximum input voltage limit in which the device operates and still has frequency-shift protection.
During short-circuit events (particularly with high-input-voltage applications), the control loop has a finite minimum
controllable on-time, and the output has a low voltage. During the switch on-time, the inductor current ramps to
the peak current limit because of the high input voltage and minimum on-time. During the switch off-time, the
inductor current would normally not have enough off-time and output voltage to ramp down by the ramp-up
amount. The frequency shift effectively increases the off-time, allowing the current to ramp down.
Selecting the Switching Frequency
The switching frequency that is selected should be the lower value of the two equations, Equation 10 and
Equation 11. Equation 10 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 11 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 11, to calculate the maximum switching frequency, one must take into account
that the output voltage decreases from the nominal voltage to 0 volts and the f
DIV
integer increases from 1 to 8,
corresponding to the frequency shift.
In Figure 38, the solid line illustrates a typical safe operating area regarding frequency shift and assumes an
output voltage of zero volts, an inductor resistance of 0.13 Ω, FET on-resistance of 0.2 Ω, and a diode voltage
drop of 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.
(10)
(11)
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