Datasheet

R
ILIM
=
20 PA
I
CL
x R
DSON-LO
LM3495
www.ti.com SNVS410F –FEBRUARY 2006–REVISED APRIL 2013
UNDER VOLTAGE LOCK-OUT
The 2.6V turn-on threshold on the voltage at VIN has a built in hysteresis of 300 mV. If input voltage drops below
2.3V the chip enters under voltage lock-out (UVLO) mode. UVLO consists of turning off both the top and bottom
FETs and remaining in that condition until input voltage rises above 2.6V.
BOOTSTRAP DIODE SELECTION
Schottky diodes are the preferred choice for the bootstrap circuit because of their low forward voltage drop. For
circuits that will operate at high ambient temperature the Schottky diode datasheet must be read carefully to
ensure that the reverse leakage current at high temperature does not increase enough to deplete the charge on
the bootstrap capacitor while the high-side FET is off. Some Schottky diodes increase their reverse leakage by
as much as 1000x at their upper temperature limit. Fast recovery and PN junction diodes maintain low reverse
leakage even at high ambient temperature. For high ambient temperature operation Schottky diodes with low
leakage across temperature or fast recovery type diodes should be used.
OVER VOLTAGE PROTECTION
The LM3495 will shut down if the output voltage exceeds 125% of the steady state target voltage for longer than
4 µs. The high-side FET is turned off and the low-side FET is turned on. The LM3495 will remain in this condition
until either the VIN pin voltage is cycled to ground, or the COMP/SD pin voltage is pulled to below 0.3V and then
released. Either of these reset mechanisms will cause the device to perform a soft-start.
LOW-SIDE CURRENT LIMIT
The current limit of the LM3495 operates by sensing the current in the low-side FET while the load current, I
O
,
circulates through it. The low-side FET drain-to-source voltage, V
DS
, is compared against the voltage of a fixed,
internal 20 µA current source and a user-selected resistor, R
ILIM
. The value of R
ILIM
for a desired current limit
threshold, I
CL
, can be selected with the following equation:
(5)
R
ILIM
is connected between the switch node and the ILIM pin. A current limit event is sensed when V
DS
exceeds
V
ILIM
. (V
ILIM
= 20 µA x R
ILIM
). The high-side switch is disabled for the following cycle and the low-side FET is kept
on during this time.
During long duration current limit conditions or a short circuit the output voltage droops. This in turn causes the
COMP/SD pin voltage to rise. If the COMP/SD pin voltage exceeds 2V and a high-side FET on-pulse is skipped,
the LM3495 increments a 4 bit counter. If 16 high-gate pulses are skipped consecutively while COMP/SD stays
above 2V, the LM3495 will enter hiccup mode. The counter is reset when the COMP/SD pin goes below 2V.
During soft-start the cycle skipping function of the low-side current limit is active, but the ability to enter hiccup
mode is disabled.
CURRENT LIMIT SENSE RESISTOR
For applications that require a higher degree of accuracy for the low-side current limit, a dedicated current sense
resistor can be added between ground and the source of the low-side FET. Figure 35 shows the circuit
connection when using a dedicated current limit sensing resistor, R
SNS
.
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