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SNVS574E – JULY 2008 – REVISED MAY 2013
START-UP REGULATOR
The LM3421/23 includes a high voltage, low dropout bias regulator. When power is applied, the regulator is
enabled and sources current into an external capacitor (C BYP ) connected to the V CC pin. The recommended
bypass capacitance for the V CC regulator is 2.2 μF to 3.3 μF. The output of the V CC regulator is monitored by an
internal UVLO circuit that protects the device from attempting to operate with insufficient supply voltage and the
supply is also internally current limited. Figure 25 shows the typical start-up waveforms for the LM3421/23.
First, C BYP is charged to be above V CC UVLO threshold (~4.2V). The C VCC charging time (t VCC ) can be estimated
as:
t VCC =
4 . 2 V
25 mA
x C BYP = 168 : x C BYP
(22)
C CMP is then charged to 0.9V over the charging time (t CMP ) which can be estimated as:
t CMP =
0.9V
25 P A
x C CMP = 36 k : x C CMP
(23)
Once C CMP = 0.9V, the part starts switching to charge C O until the LED current is in regulation. The C O charging
time (t CO ) can be roughly estimated as:
t CO = C O x
V O
I LED
(24)
The system start-up time (t SU ) is defined as:
t SU = t VCC + t CMP + t CO
(25)
In some configurations, the start-up waveform will overshoot the steady state COMP pin voltage. In this case, the
LED current and output voltage will overshoot also, which can trip the over-voltage or protection, causing a race
condition. The easiest way to prevent this is to use a larger compensation capacitor (C CMP ), thereby slowing
down the control loop.
OVER-VOLTAGE LOCKOUT (OVLO)
V O
LM3421/23
23 P A
R OV2
OVP
1.24V
OVLO
R OV1
Figure 26. Over-Voltage Protection Circuitry
The LM3421/23 can be configured to detect an output (or input) 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 Figure 26 . When the
OVLO threshold is 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 O ) should be sensed and translated to ground by using a single PNP as
The over-voltage turn-off threshold (V TURN-OFF ) is defined:
Ground Referenced
V TURN - OFF = 1 . 24V x ¨ ¨ OV 1
§ R + R OV 2 ·
?
? R OV 1 1
?
(26)
Copyright ? 2008–2013, Texas Instruments Incorporated
19
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