XR77103 Datasheet, PDF (17/28 Pages) Exar Corporation – 3-Output Programmable Buck Regulator

XR77103
Applications Information (Continued)
Synchronization
The status of the SYNC pin will be ignored during
start-up and the XR77103’s control will only synchronize
to an external signal after 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 AGND.
Although the device can lock to external clock running up
to 2.31MHz, doing this will alter the start-up times, start-up
delays and PGOOD delays, and there will be higher losses
than what is shown in Figures 22-33. For details contact
[e-mail].
Out-of-Phase Operation
All converters operate in phase, or one converter operates
180 degrees out-of-phase with the other two converters
(see Figure 38). The phase shift among the converter is
programmable. The bits 6, 5 of the register 09h are allocated
for this feature. This enables the system, having less input
ripple, to lower component cost, save board space and
reduce EMI.
LX1
LX2
LX3
All converters in phase
LX1
LX2
LX3
Buck3 and Buck1/2 are 180° out-of-phase
LX1
LX2
LX3
Buck1 and Buck2/3 are 180° out-of-phase
LX1
LX2
LX3
Buck2 and Buck1/3 are 180° out-of-phase
Two Buck Regulators in Parallel Operation
(Current Sharing)
The XR77103 can be used in parallel operation to increase
output current capacity. Figure 39 shows one of possible
configurations. To enable this a user needs:
Hardware Configuration
a) To connect both V_OUTx together.
b) To connect both C_OMPx together.
Software Configuration
a) To set 180 out-of-phase operation between
buck regulators (register 09h).
b) Programming both V_OUTx to the same output.
Then, two out of three bucks will run in parallel and load
current is shared in average.
The ideal case is to use Buck 2 and Buck 3 in parallel
operation since they are both identical in design.
BUCK2
LX2
LX2
VOUT2
COMP2
VOUT
BUCK3
COMP3
LX3
LX3
VOUT3
Figure 38. Out-of-Phase Operation
Figure 39. Parallel Operation
REV1B
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