LM3423 Datasheet, PDF (2/10 Pages) National Semiconductor (TI) – The boost circuit can be easily redesigned for different

Pin Descriptions
LM3423
1
LM3421
1
Name
V_IN
2
2
EN
3
3
COMP
4
4
CSH
5
5
RCT
6
6
AGND
7
7
OVP
8
8
nDIM
9
-
FLT
10
-
TIMR
11
-
LRDY
12
-
DPOL
13
9
DDRV
14
10
PGND
15
11
GATE
16
12
V_CC
17
13
IS
18
14
19
15
20
DAP (21)
16
DAP (17)
RPD
HSP
HSN
DAP
Description
Input Voltage
Enable
Compensation
Current Sense High
Resistor Capacitor Timing
Analog Ground
Over-Voltage Protection
Dimming Input /
Under-Voltage Protection
Fault Flag
Fault Timer
LED Ready Flag
Dim Polarity
Dim Gate Drive Output
Power Ground
Main Gate Drive Output
Internal Regulator Output
Main Switch Current Sense
Resistor Pull Down
LED Current Sense Positive
LED Current Sense Negative
Thermal PAD on bottom of IC
Function
Bypass with 100 nF capacitor to AGND as close to the
device as possible in the circuit board layout.
Connect to AGND for zero current shutdown or apply >
2.4V to enable device.
Connect a capacitor to AGND to set the compensation.
Connect a resistor to AGND to set the signal current.
For analog dimming, connect a controlled current
source or a potentiometer to AGND as detailed in the
Analog Dimming section.
External RC network sets the predictive “off-time” and
thus the switching frequency.
Connect to PGND through the DAP copper pad to
provide ground return for CSH, COMP, RCT, and TIMR.
Connect to a resistor divider from V_O to program output
over-voltage lockout (OVLO). Turn-off threshold is
1.24V and hysteresis for turn-on is provided by 23 µA
current source.
Connect a PWM signal for dimming as detailed in the
PWM Dimming section and/or a resistor divider from
V_IN to program input under-voltage lockout (UVLO).
Turn-on threshold is 1.24V and hysteresis for turn-off is
provided by 23 µA current source.
Connect to pull-up resistor from VIN and N-channel
MosFET open drain output is high when a fault condition
is latched by the timer.
Connect a capacitor to AGND to set the time delay
before a sensed fault condition is latched.
Connect to pull-up resistor from VIN and N-channel
MosFET open drain output pulls down when the LED
current is not in regulation.
Connect to AGND if dimming with a series P-channel
MosFET or leave open when dimming with series N-
channel MosFET.
Connect to the gate of the dimming MosFET.
Connect to AGND through the DAP copper pad to
provide ground return for GATE and DDRV.
Connect to the gate of the main switching MosFET.
Bypass with 2.2 µF–3.3 µF ceramic capacitor to PGND.
Connect to the drain of the main N-channel MosFET
switch for R_DS-ON sensing or to a sense resistor installed
in the source of the same device.
Connect the low side of all external resistor dividers
(V_IN UVLO, OVP) to implement “zero-current”
shutdown.
Connect through a series resistor to the positive side of
the LED current sense resistor.
Connect through a series resistor to the negative side
of the LED current sense resistor.
Star ground, connecting AGND and PGND.
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