14
Rev. C
For more information
supply is current limited, or has a relatively high source
resistance. A switching regulator draws constant power
from the source, so source current 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 LT8607 will remain off until V
IN
is above V
IN(EN)
.
Due to the comparator’s hysteresis, switching 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
LT8607. Therefore, the V
IN(EN)
resistors 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 cur-
rent for the LT8607’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 temperature because of the
higher power dissipation across the LDO. Do not connect
an external load to the INTV
CC
pin.
Output Voltage Tracking and Soft-Start (MSOP Only)
The LT8607 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 current 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
APPLICATIONS INFORMATION
can be externally driven by another voltage source. From
0V to 0.778V, the TR/SS voltage will override the internal
0.778V reference input to the error amplifier, thus regulat-
ing the FB pin voltage to that of TR/SS pin. When TR/SS
is above 0.778V, tracking is disabled and the feedback
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
volt-
age falling too low, or thermal shutdown. The LT8607 and
LT8607B DFN does not have the TR/SS pin or functionality.
Output Power Good
When the LT8607’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 glitching 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 (MSOP Only)
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 LT8607 oscillator to an external
frequency connect a square wave (with 20% to 80% duty
cycle) to the SYNC pin. The square wave amplitude should
have valleys that are below 0.9V and peaks above 2.7V
(up to 5V).
The LT8607 will not enter Burst Mode operation at low
output loads while synchronized to an external clock, but
instead will pulse skip to maintain regulation. The LT8607
may be synchronized over a 200kHz to 2.2MHz range. The
R
T
resistor should be chosen to set the LT8607 switch-
ing frequency equal to or below the lowest synchroni-
zation input. For example, if the synchronization signal
will be 500kHz and higher, the R
T
should be selected for