XR77128 Datasheet, PDF (26/28 Pages) Exar Corporation – Quad Output Digital PWM/PFM

XR77128
reset (if the Automatically Reset After Flashing box is
checked), and boot the design configuration from the Flash.
proper thermal design becomes critical. For lower power
levels using properly sized MOSFETs, the use of the inter-
nal 5V regulator as a gate drive supply may be appropriate.
Layout Guidelines
Refer to application note ANP-32 “Practical Layout Guide-
lines for PowerXR Designs”.
For users that wish to create their own programming proce-
dure so they can re-program Flash in-circuit using their sys-
tem software, they should contact Exar for a list of I²C Flash
Commands needed or refer to ANP-38.
Enabling XR77128
XR77128 has an internal pull-up ensuring it gets enabled
as soon as internal voltage supplies have ramped up and
are in regulation.
Driving the Enable pin low externally will keep the controller
in the shut-down mode. A simple open drain pull down is
the recommended way to shut XR77128 down.
If the Enable pin is driven high externally to control
XR77128 coming out of the shut-down mode, care must be
taken in such a scenario to ensure the Enable pin is driven
high after VCC gets supplied to the controller.
A diode in series with a resistor between the LDO5 and
ENABLE pins may offer a way to more quickly pull down the
LDO5 output when the ENABLE pin is pulled low.
In the application where VCC = LDO5 = 4.75V to 5.5V, dis-
abling the device by pulling the ENABLE pin low is not rec-
ommended. It is recommended to leave the ENABLE pin
floating and place the controller in the Standby Mode
instead. The Standby Mode is defined as the state when all
switching channels and LDOOUT are disabled, all IOs pro-
grammed as inputs, and the system clock is disabled. In
this state the device consumes 550uA typical.
Application Information
Thermal Design
As a four channel controller with internal MOSFET drivers
and 5V gate drive supply all in one 7x7mm 44pin TQFN
package, there is the potential for the power dissipation to
exceed the package thermal limitations. The XR77128 has
an internal LDO which supplies 5V to the internal circuitry
and MOSFET drivers during startup. It is generally
expected that either one of the switching regulator outputs
is 5V or another 5V rail is available in the system and con-
nected to the V5EXT pin. If there is no 5V available in the
system, then the power loss will increase significantly and
© 2014 Exar Corporation
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exar.com/XR77128
Rev 1A