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| LM3424 Datasheet, PDF (20/69 Pages) National Semiconductor (TI) – Constant Current N-Channel Controller with Thermal Foldback for Driving LEDs | |||
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LM3424, LM3424-Q1
SNVS603C â AUGUST 2009 â REVISED AUGUST 2015
Feature Description (continued)
www.ti.com
80
90
öP2
60
45
40
GAIN
20
öZ1
0
PHASE
-20
60° Phase Margin
-40
öP1
öP3
0
-45
-90
-135
-180
-60
-225
-80
1e-1
1e1
1e3
1e5
FREQUENCY (Hz)
-270
1e7
Figure 26. Compensated Loop Gain Frequency Response
7.3.8 Start-Up Regulator and Soft-Start
The LM3424 includes a high voltage, low dropout bias regulator. When power is applied, the regulator is enabled
and sources current into an external capacitor (CBYP) connected to the VCC pin. The recommended bypass
capacitance for the VCC regulator is 2.2 µF to 3.3 µF. The output of the VCC 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.
The LM3424 also has programmable soft-start, set by an external capacitor (CSS), connected from SS to GND.
For CSS to affect start-up, CREF > CNTC must be maintained so that the converter does not start in foldback mode.
Figure 27 shows the typical start-up waveforms for the LM3424 assuming CREF > CNTC.
VCMP
0.9V
0
tVCC tCMP tCO
0.9V
0.7V
0
t
tVCC tCMP-SS tSS tCO
Figure 27. Start-up Waveforms
First, CBYP is charged to be above VCC UVLO threshold (~4.2V). The CVCC charging time (tVCC) can be estimated
as:
t VCC
=
4.2V
25 mA
x CBYP =168:
x CBYP
(21)
Assuming there is no CSS or if CSS is less than 40% of CCMP , CCMP is then charged to 0.9V over the charging
time (tCMP) which can be estimated as:
tCMP
=
0.9V
25 PA
x
CCMP = 36 k: x CCMP
(22)
20
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Product Folder Links: LM3424 LM3424-Q1
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