LT3688 LINER [Linear Technology], LT3688 Datasheet - Page 10

• Manufacturer Part Number: LT3688
• Description: Dual 800mA Step-Down Switching Regulator with Power-On Reset
• Manufacturer: LINER [Linear Technology]

OPERATION
LT3688
The LT3688 is a constant-frequency, current mode step-
down regulator with two reset timers and a watchdog
timer that perform microprocessor supervisory functions.
Operation can be best understood by referring to the Block
Diagram. Keeping the EN/UVLO pin at ground completely
shuts off the part drawing minimal current from the V
source. To turn on the internal bandgap and the rest of the
logic circuitry, raise the EN/UVLO pin above the accurate
threshold of 1.25V. Also, V
for the part to start switching.
Switching Regulator Operation
An oscillator, with frequency set by R
fl op, turning on the internal power switch. An amplifi er
and comparator monitor the current fl owing between the
V
rent reaches a level determined by the voltage at V
error amplifi er measures the output voltage through an
external resistor divider tied to the FB pin and servos the
V
current is delivered to the output; if it decreases, less
current is delivered. An active clamp on the V
provides current limit. The V
by the internal soft-start circuit during start-up or after a
fault condition takes place.
An internal regulator provides power to the control cir-
cuitry. The internal regulator normally draws current from
the V
voltage higher than 3V, bias current will be drawn from the
external source (typically the regulated output voltage).
This improves effi ciency. The BIAS pin also provides a
current path to the internal boost diode that charges up
the boost capacitor. The switch driver operates either from
the V
to generate a voltage at the BST pin that is higher than
the V
internal NPN power switch for effi cient operation. To further
10
IN
C
voltage. If the error amplifi er’s output increases, more
and SW pins, turning the switch off when this cur-
IN
IN
IN
pin, but if the BIAS pin is connected to an external
or from the BST pin. An external capacitor is used
supply. This allows the driver to fully saturate the
IN
needs to be higher than 3.5V
C
voltage is also controlled
T
, enables an RS fl ip
C
voltage
C
. An
IN
optimize effi ciency, the LT3688 automatically switches to
Burst Mode operation in light load situations. Between
bursts, all circuitry associated with controlling the output
switch is shut down, reducing the input supply current to
115μA in a typical application.
A comparator monitors the current fl owing through the
catch diode via the DA pin. This comparator delays switch-
ing if the diode current goes higher than 1.2A (typical)
during a fault condition such as a shorted output with high
input voltage. Switching will only resume once the diode
current has fallen below the 1.2A limit. This way the DA
comparator regulates the valley current of the inductor
to 1.2A during short circuit. This will ensure that the part
will survive a short-circuit event.
Power-On Reset and Watchdog Timer Operation
The LT3688 has two power-on reset comparators that
monitor the regulated output voltages. If V
below the regulation value, the RST pin is pulled low. Once
the output voltage crosses over 90% of the regulation
value, a reset timer is started and RST is released after
the programmed reset delay time. The reset delay is
programmable through the C
The watchdog typically monitors a microprocessor’s
activity. The watchdog can be enabled or disabled by ap-
plying a logic signal to the WDE pin. The watchdog timer
requires successive negative edges on the WDI pin to
come within a programmed time window to keep WDO
from going low. If the time between the two negative WDI
edges is too short or too long, then the WDO pin will be
pulled low. When the WDO pin goes low, it stays low for
a time period equivalent to 1/8th of the watchdog window
upper boundary. The WDO pin will go high again once the
timer expires or if the RST pin goes low. The watchdog
window upper and lower boundaries can be set through
the C
WDT
pin.
POR
pin.
OUT
is 10%
3688f