Datasheet

R k f(MHz)
OSC
( ) = 174W ·
-1.122
TPS65251
www.ti.com SLVSAA4D –JUNE 2010–REVISED DECEMBER 2012
DETAILED DESCRIPTION
Adjustable Switching Frequency
To select the internal switching frequency connect a resistor from ROSC to ground. Figure 21 shows the required
resistance for a given switching frequency.
Figure 21. ROSC vs Switching Frequency
(1)
For operation at 800 kHz a 230-kΩ resistor is required.
Synchronization
The status of the SYNC pin will be ignored during start-up and the TPS65251’s control will only synchronize to
an external signal after the PGOOD signal is asserted. The status of the SYNC pin will be ignored during start-up
and the TPS65251 will only synchronize to an external clock if the PGOOD signal is asserted. When
synchronization is applied, the PWM oscillator frequency must be lower than the sync pulse frequency to allow
the external signal trumping the oscillator pulse reliably. When synchronization is not applied, the SYNC pin
should be connected to ground.
Out-of-Phase Operation
Buck 1 has a low conduction resistance compared to Buck 2 and 3. Normally Buck 1 is used to drive higher
system loads. Buck 2 and 3 are used to drive some peripheral loads like I/O and line drivers . The combination of
Buck 2 and 3’s loads may be on par with Buck 1’s. In order to reduce input ripple current, Buck 2 operates in
phase with Buck 3; Buck 1 and Buck 2 operate 180 degrees out-of-phase. This enables the system, having less
input ripple, to lower component cost, save board space and reduce EMI.
Delayed Start-Up
If a delayed start-up is required on any of the buck converters fit a ceramic capacitor to the ENx pins. The delay
added is ~1.67 ms per nF connected to the pin. Note that the EN pins have a weak 1-µA pull-up to the 3V3 rail.
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