Datasheet

LT8640/LT8640-1
21
Rev.C
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APPLICATIONS INFORMATION
Ceramic Capacitors
Ceramic capacitors are small, robust and have very low
ESR. However, ceramic capacitors can cause problems
when used with the LT8640/LT8640-1 due to their
piezoelectric nature. When in Burst Mode operation, the
LT8640/LT8640-1’s switching frequency depends on the
load current, and at very light loads the LT8640/LT8640-
1 can excite the ceramic capacitor at audio frequencies,
generating audible noise. Since the LT8640/LT8640-1
operates at a lower current limit during Burst Mode op
-
eration, the noise is typically very quiet to a casual ear. If
this is unacceptable, use a high performance tantalum or
electrolytic capacitor at the output. Low noise ceramic
capacitors are also available.
A final precaution regarding ceramic capacitors concerns the
maximum input voltage rating of the LT8640/LT8640-1. As
previously mentioned, a ceramic input capacitor combined
with trace or cable inductance forms a high quality (un
-
derdamped) tank circuit. If the LT8640/LT8640-1 circuit
is plugged into a live supply, the input voltage can ring to
twice its nominal value, possibly exceeding the LT8640/
LT8640-1’s rating. This situation is easily avoided (see
Linear Technology Application Note 88).
Enable Pin
The LT8640/LT8640-1 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
LT8640/LT8640-1 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
(11)
where the LT8640/LT8640-1 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 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 LT8640/LT8640-1. 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 LT8640/LT8640-1’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
LT8640/LT8640-1, 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.
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