Datasheet
1.24V
20 PA
LM3424
R
OV2
R
OV1
V
O
OVLO
OVP
COSSSSCMPVCCSSSU
ttttt
+++=
--
COCMPVCCSU
tttt
++=
SSSSSS
C20 kCt x:
=
x
=
PA
10
V2.0
CMPCMPCMP - SS
C28Ct x
k:
=
x
=
PA
25
0.70V
OCO
Ct
x
=
O
V
LED
I
CMPCMP
CMP
C36Ct x
k:
=
x
=
PA
25
0.9V
BYPBYPVCC
C168Ct x:
=
x
=
mA25
V2.4
LM3424
SNVS603B –AUGUST 2009–REVISED OCTOBER 2009
www.ti.com
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 (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.
The LM3424 also has programmable soft-start, set by an external capacitor (C
SS
), connected from SS to GND.
For C
SS
to affect start-up, C
REF
> C
NTC
must be maintained so that the converter does not start in foldback mode.
Figure 30 shows the typical start-up waveforms for the LM3424 assuming C
REF
> C
NTC
.
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:
(22)
Assuming there is no C
SS
or if C
SS
is less than 40% of C
CMP
, C
CMP
is then charged to 0.9V over the charging
time (t
CMP
) which can be estimated as:
(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:
(24)
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:
(25)
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:
(26)
The system start-up time (t
SU
or t
SU-SS
) is defined as:
C
SS
< 0.4 x C
CMP
(27)
C
SS
> 0.4 x C
CMP
(28)
As a general rule of thumb, standard smooth startup operation can be achieved with C
SS
= C
CMP
.
OVER-VOLTAGE LOCKOUT (OVLO)
Figure 31. Over-Voltage Protection Circuitry
22 Submit Documentation Feedback Copyright © 2009, Texas Instruments Incorporated
Product Folder Links: LM3424