LM3423MHBSTEVAL National Semiconductor, LM3423MHBSTEVAL Datasheet - Page 16

• Manufacturer Part Number: LM3423MHBSTEVAL
• Description: BOARD, EVALUATION, FOR LM3423MHBST
• Manufacturer: National Semiconductor

Specifications of LM3423MHBSTEVAL

• Kit Contents: LM3423 Assembled Evaluation Board, LM3423 Datasheet, Application Note
• Svhc: No SVHC (15-Dec-2010)
• Kit Features: Cycle-by-Cycle Current Limit, LED Ready Flag, Fault Flag, Programmable

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OVER-VOLTAGE LOCKOUT (OVLO)
The LM3421/23 can be configured to detect an output (or in-
put) over-voltage condition via the OVP pin. The pin features
a precision 1.24V threshold with 23 µA (typical) of hysteresis
current as shown in
exceeded, the GATE pin is immediately pulled low and a 23
µA current source provides hysteresis to the lower threshold
of the OVLO hysteretic band.
If the LEDs are referenced to a potential other than ground
(floating), as in the buck-boost and buck configuration, the
output voltage (V
ground by using a single PNP as shown in
The over-voltage turn-off threshold (V
Ground Referenced
Floating
In the ground referenced configuration, the voltage across
R
V
The over-voltage hysteresis (V
OV2
O
- 620 mV where 620 mV approximates V
is V
FIGURE 12. Over-Voltage Protection Circuitry
FIGURE 13. Floating Output OVP Circuitry
O
- 1.24V whereas in the floating configuration it is
O
) should be sensed and translated to
Figure
12. When the OVLO threshold is
HYSO
) is defined:
TURN-OFF
30067359
Figure
BE
) is defined:
of the PNP.
13.
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16
The OVLO feature can cause some interesting results if the
OVLO trip-point is set too cose to V
has a modest amount of voltage overshoot before the control
loop gains control of I
threshold, the controller shuts down, opening the dimming
MosFET. This isolates the LED load from the converter and
the output capacitance. The voltage will then discharge very
slowly through the HSP and HSN pins until V
the lower threshold, where the process repeats. This looks
like the LEDs are blinking at around 2 Hz. This mode can be
escaped if the input voltage is reduced.
INPUT UNDER-VOLTAGE LOCKOUT (UVLO)
The nDIM pin is a dual-function input that features an accurate
1.24V threshold with programmable hysteresis as shown in
Figure
for the LEDs and as a V
and exceeds the 1.24V threshold, 23 µA (typical) of current is
driven out of the nDIM pin into the resistor divider providing
programmable hysteresis.
When using the nDIM pin for UVLO and PWM dimming con-
currently, the UVLO circuit can have an extra series resistor
to set the hysteresis. This allows the standard resistor divider
to have smaller resistor values minimizing PWM delays due
to a pull-down MosFET at the nDIM pin (see PWM Dimming
section). In general, at least 3V of hysteresis is preferable
when PWM dimming, if operating near the UVLO threshold.
The turn-on threshold (V
The hysteresis (V
UVLO only
PWM dimming and UVLO
When "zero current" shutdown and UVLO are implemented
together, the EN pin can be used to escape UVLO. The nDIM
pin will pull-up to V
within the UVLO hysteretic window when EN is pulled high
again, the controller will start-up even though V
exceeded.
14. This pin functions as both the PWM dimming input
FIGURE 14. UVLO Circuit
HYS
IN
when EN is pulled low, therefore if V
) is defined as follows:
LED
IN
. If the overshoot exceeds the OVLO
TURN-ON
UVLO. When the pin voltage rises
) is defined as follows:
O
. At turn-on, the converter
O
TURN-ON
drops below
300673a5
is not
IN
is