Datasheet

TPS54262-Q1
www.ti.com SLVS996C –SEPTEMBER 2009–REVISED JUNE 2010
Enable and Shutdown
The EN pin (pin 5) provides electrical ON/OFF control of the regulator. Once the EN pin voltage exceeds the
upper threshold voltage (V
IH
), the regulator starts operating and the internal soft start begins to ramp. If the EN
pin voltage is pulled below the lower threshold voltage (V
IL
), the regulator stops switching and the internal soft
start resets. Connecting this pin to ground or to any voltage less than V
IL
disables the regulator and causes the
device to shut down. This pin must have an external pullup or pulldown to change the state of the device.
Soft Start
An external soft start capacitor is connected to SS pin (pin 11) to set the minimum time to reach the desired
regulated output voltage (V
Reg
) during power up cycle. This prevents the output voltage from overshooting when
the device is powered up. This is also useful when the load requires a controlled voltage slew rate, and also
helps to limit the current drawn from the input voltage supply line.
For proper operation, the following conditions must be satisfied during power-up and after a short circuit event:
• VIN – V
Reg
> 2.5 V
• Load current < 1 A, until RSTgoes high
The current limit foldback is released after the feedback voltage (at VSENSE pin) is high enough such that RST
is asserted high. The recommended value of soft start capacitor is 100 nF (typical) for startup load current of 1 A
(maximum).
Oscillator Frequency
The oscillator frequency can be set by connecting an external resistor (R8 in Figure 4) to RT pin (pin 6) .
Figure 23 shows the relation between the resistor value (RT) and switching frequency (f
sw
). The switching
frequency can be set in the range 200 kHz to 2200 kHz. In addition, the switching frequency can be imposed
externally by a clock signal (f
ext
) at the SYNC pin.
Selecting the Switching Frequency
A power supply switching at a higher switching frequency allows use of lower value inductor and smaller output
capacitor compared to a power supply that switches at a lower frequency. Typically, the user will want to choose
the highest switching frequency possible since this will produce the smallest solution size. The switching
frequency that can be selected is limited by the following factors:
• The input voltage
• The minimum target regulated voltage
• Minimum on-time of the internal switching transistor
• Frequency shift limitation
Selecting lower switching frequency results in using an inductor and capacitor of a larger value, where as
selecting higher switching frequency results in higher switching and gate drive power losses. Therefore, a
tradeoff has to be made between physical size of the power supply and the power dissipation at the system/
application level.
The minimum and maximum duty cycles can be expressed in terms of input and output voltage as shown in
Equation 6.
(6)
Where,
D
Min
= minimum duty cycle
D
Max
= maximum duty cycle
VIN
Min
= minimum input voltage
VIN
Max
= maximum input voltage
V
Reg-Min
= minimum regulated output voltage
V
Reg-Max
= maximum regulated output voltage
From Equation 6, maximum switching frequency can be calculated in Equation 7.
Copyright © 2009–2010, Texas Instruments Incorporated Submit Documentation Feedback 17
Product Folder Link(s): TPS54262-Q1