Datasheet

LEDDIMLEDDIM
IDI x
=
-
t
OFF
i
LED
(t)
I
LED-MAX
t
I
DIM-LED
0
D
DIM
x T
DIM
T
DIM
I
LED
LM3409, LM3409HV, LM3409-Q1
SNVS602J –MARCH 2009–REVISED MAY 2013 www.ti.com
Figure 27. LED Current i
LED
(t) During EN Pin PWM Dimming
PWM DIMMING USING THE EN PIN
The enable pin (EN) is a TTL compatible input for PWM dimming of the LED. A logic low (below 0.5V) at EN will
disable the internal driver and shut off the current flow to the LED array. While the EN pin is in a logic low state
the support circuitry (driver, bandgap, V
CC
regulator) remains active in order to minimize the time needed to turn
the LED array back on when the EN pin sees a logic high (above 1.74V).
Figure 27 shows the LED current (i
LED
(t)) during PWM dimming where duty cycle (D
DIM
) is the percentage of the
dimming period (T
DIM
) that the PFET is switching. For the remainder of T
DIM
, the PFET is disabled. The resulting
dimmed average LED current (I
DIM-LED
) is:
(15)
The LED current rise and fall times (which are limited by the slew rate of the inductor as well as the delay from
activation of the EN pin to the response of the external PFET) limit the achievable T
DIM
and D
DIM
. In general,
dimming frequency should be at least one order of magnitude lower than the steady state switching frequency in
order to prevent aliasing. However, for good linear response across the entire dimming range, the dimming
frequency may need to be even lower.
HIGH VOLTAGE NEGATIVE BIAS REGULATOR
The LM3409/09HV contains an internal linear regulator where the steady state VCC pin voltage is typically 6.2V
below the voltage at the VIN pin. The VCC pin should be bypassed to the VIN pin with at least 1µF of ceramic
capacitance connected as close as possible to the IC.
INPUT UNDER-VOLTAGE LOCKOUT (UVLO)
Under-voltage lockout is set with a resistor divider from V
IN
to GND and is compared against a 1.24V threshold
as shown in Figure 28. Once the input voltage is above the preset UVLO rising threshold (and assuming the part
is enabled), the internal circuitry becomes active and a 22µA current source at the UVLO pin is turned on. This
extra current provides hysteresis to create a lower UVLO falling threshold. The resistor divider is chosen to set
both the UVLO rising and falling thresholds.
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