LM3424 Datasheet, PDF (21/69 Pages) National Semiconductor (TI) – Constant Current N-Channel Controller with Thermal Foldback for Driving LEDs

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LM3424, LM3424-Q1
SNVS603C – AUGUST 2009 – REVISED AUGUST 2015
Feature Description (continued)
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
(23)
If C_SS is greater than 40% of C_CMP, the compensation capacitor will only charge to 0.7V over a smaller C_CMP
charging time (t_CMP-SS) which can be estimated as:
t_{CMP - SS}
=
0.70V
25 PA
x
C_CMP = 28 k:
x C_CMP
(24)
Then COMP will clamp to SS, forcing COMP to rise (the last 200 mV before switching begins) according to the
C_SS charging time (t_SS) which can be estimated as:
t _SS
=
0.2V
10 PA
x
C_SS
=
20
k:
x
C_SS
(25)
The system start-up time (t_SU or t_SU-SS) is defined as:
C_SS < 0.4 x C_CMP
t_SU = t _VCC + t_CMP + t_CO
(26)
C_SS > 0.4 x C_CMP
t_{SU- SS} = t _VCC + t_{CMP- SS} + t_SS + t_CO
(27)
As a general rule of thumb, standard smooth startup operation can be achieved with C_SS = C_CMP.
7.3.9 Overvoltage Lockout (OVLO)
The LM3424 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 20 µA (typical) of hysteresis current as shown in Figure 28. When the
OVLO threshold is exceeded, the GATE pin is immediately pulled low and a 20 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
shown in Figure 29.
The over-voltage turn-off threshold (V_TURN-OFF) is defined:
Ground Referenced
V_{TURN- OFF} = 1.24V x _{¨¨©§}^ROV_R^{1 +}_OV^R₁ ^OV2_{¸¸¹·}
(28)
Floating
V_{TURN -}
OFF
=
1.24V
x
¨¨©§^0.5
x
R_OV1+
R_OV1
R _{OV2¸¸¹·}
(29)
In the ground referenced configuration, the voltage across R_OV2 is V_O - 1.24 V whereas in the floating
configuration it is V_O - 620 mV where 620 mV approximates V_BE of the PNP.
The over-voltage hysteresis (V_HYSO) is defined:
V_HYSO = 20 2A x R_OV2
(30)
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