Datasheet

LT8609/LT8609A/LT8609B
17
Rev. H
For more information www.analog.com
feature is not used, or tied to a logic level if shutdown
control is required.
Adding a resistor divider from V
IN
to EN programs the
LT8609/LT8609A/LT8609B to regulate the output only
when V
IN
is above a desired voltage (see Block Diagram).
Typically, this threshold, V
IN(EN)
, is used in situations
where the input supply is current limited, or has a rel-
atively high source resistance. A switching regulator
draws constant power from the source, so source cur-
rent increases as source voltage drops. This looks like
a negative resistance load to the source and can cause
the source to current limit or latch low under low source
voltage conditions. The V
IN(EN)
threshold prevents the
regulator from operating at source voltages where the
problems might occur. This threshold can be adjusted by
setting the values R3 and R4 such that they satisfy the
following equation:
V
IN(EN)
=
R3
R4
+1
⎛
⎝
⎜
⎞
⎠
⎟
• 1V
where the LT8609/LT8609A/LT8609B will remain off until V
IN
is above V
IN(EN)
. Due to the comparator’s hysteresis, switch-
ing will not stop until the input falls slightly below V
IN(EN)
.
When in Burst Mode operation for light-load currents,
the current through the V
IN(EN)
resistor network can eas-
ily be greater than the supply current consumed by the
LT8609/LT8609A/LT8609B. Therefore, the V
IN(EN)
resis-
tors should be large to minimize their effect on efficiency
at low loads.
INTV
CC
Regulator
An internal low dropout (LDO) regulator produces the
3.5V supply from V
IN
that powers the drivers and the
internal bias circuitry. The INTV
CC
can supply enough
current for the LT8609/LT8609A/LT8609B’s circuitry
and must be bypassed to ground with a minimum of
1μF ceramic capacitor. Good bypassing is necessary to
supply the high transient currents required by the power
MOSFET gate drivers. Applications with high input voltage
and high switching frequency will increase die tempera-
ture because of the higher power dissipation across the
LDO. Do not connect an external load to the INTV
CC
pin.
APPLICATIONS INFORMATION
Output Voltage Tracking and Soft-Start
The LT8609/LT8609A/LT8609B allows the user to
program its output voltage ramp rate by means of
the TR/SS pin. An internal 2μA pulls up the TR/SS
pin to INTV
CC
. Putting an external capacitor on TR/
SS enables soft-starting the output to prevent cur-
rent surge on the input supply. During the soft-start
ramp the output voltage will proportionally track the
TR/SS pin voltage. For output tracking applications, TR/
SS can be externally driven by another voltage source.
From 0V to 0.782V, the TR/SS voltage will override the
internal 0.782V reference input to the error amplifier, thus
regulating the FB pin voltage to that of TR/SS pin. When
TR/SS is above 0.782V, tracking is disabled and the feed-
back voltage will regulate to the internal reference voltage.
An active pull-down circuit is connected to the TR/SS pin
which will discharge the external soft-start capacitor in
the case of fault conditions and restart the ramp when the
faults are cleared. Fault conditions that clear the soft-start
capacitor are the EN/UV pin transitioning low, V
IN
voltage
falling too low, or thermal shutdown.
Output Power Good
When the LT8609/LT8609A/LT8609B’s output voltage is
within the ±8.5% window of the regulation point, which is
a V
FB
voltage in the range of 0.716V to 0.849V (typical),
the output voltage is considered good and the open-drain
PG pin goes high impedance and is typically pulled high
with an external resistor. Otherwise, the internal drain pull-
down device will pull the PG pin low. To prevent glitch-
ing both the upper and lower thresholds include 0.5% of
hysteresis.
The PG pin is also actively pulled low during several fault
conditions: EN/UV pin is below 1V, INTV
CC
has fallen too
low, V
IN
is too low, or thermal shutdown.
Synchronization
To select low ripple Burst Mode operation, tie the SYNC
pin below 0.4V (this can be ground or a logic low out
-
put). To synchronize the LT8609/LT8609A oscillator to
an external frequency connect a square wave (with 20%
to 80% duty cycle) to the SYNC pin. The square wave
Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.