LT8610 Datasheet, PDF (15/20 Pages) Linear Technology – 42V, 2.5A Synchronous Step-Down Regulator

LT8610
APPLICATIONS INFORMATION
A final precaution regarding ceramic capacitors concerns
the maximum input voltage rating of the LT8610. As
previously mentioned, a ceramic input capacitor combined
with trace or cable inductance forms a high quality (un-
derdamped) tank circuit. If the LT8610 circuit is plugged
into a live supply, the input voltage can ring to twice its
nominal value, possibly exceeding the LT8610’s rating.
This situation is easily avoided (see Linear Technology
Application Note 88).
Enable Pin
The LT8610 is in shutdown when the EN pin is low and
active when the pin is high. The rising threshold of the EN
comparator is 1.0V, with 40mV of hysteresis. The EN pin
can be tied to V_IN if the shutdown 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
LT8610 to regulate the output only when V_IN is above a
desired voltage (see the Block Diagram). Typically, this
threshold, V_IN(EN), is used in situations where the input
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^{⎞⎠⎟}
•
1.0V
(10)
where the LT8610 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 operating in Burst Mode operation for light load
currents, the current through the V_IN(EN) resistor network
can easily be greater than the supply current consumed
by the LT8610. 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 3.4V
supply from V_IN that powers the drivers and the internal
bias circuitry. The INTV_CC can supply enough current for
the LT8610’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. To improve efficiency
the internal LDO can also draw current from the BIAS
pin when the BIAS pin is at 3.1V or higher. Typically the
BIAS pin can be tied to the output of the LT8610, or can
be tied to an external supply of 3.3V or above. If BIAS is
connected to a supply other than V_OUT, be sure to bypass
with a local ceramic capacitor. If the BIAS pin is below
3.0V, the internal LDO will consume current from V_IN.
Applications with high input voltage and high switching
frequency where the internal LDO pulls current from V_IN
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
The LT8610 allows the user to program its output voltage
ramp rate by means of the TR/SS pin. An internal 2.2μA
pulls up the TR/SS pin to INTV_CC. Putting an external
capacitor on TR/SS enables soft starting the output to pre-
vent 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 can
be externally driven by another voltage source. From 0V to
0.97V, the TR/SS voltage will override the internal 0.97V
reference input to the error amplifier, thus regulating the
FB pin voltage to that of TR/SS pin. When TR/SS is above
0.97V, tracking is disabled and the feedback voltage will
regulate to the internal reference voltage. The TR/SS pin
may be left floating if the function is not needed.
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.
8610p
15