LM3429_14 Datasheet, PDF (16/50 Pages) Texas Instruments – LM3429Q1 N-Channel Controller for Constant Current LED Drivers

LM3429
SNVS616F – APRIL 2009 – REVISED JANUARY 2010
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Z_P3
=
R_FS
1
x
C_FS
(18)
The total system transfer function becomes:
¨¨©§^1- _Z^s_Z1¸¸¹·
T = T_U0 x _{¨¨©§1+ ZsP1¸¸¹· x ¨¨©§1+ ZsP2¸¸¹· x ¨¨©§1+ ZsP3¸¸¹·}
(19)
The resulting compensated loop gain frequency response shown in Figure 10 indicates that the system has
adequate phase margin (above 45°) if the dominant compensation pole is placed low enough, ensuring stability:
80
90
ö_P2
60
45
40
GAIN
20
ö_Z1
0
PHASE
-20
60° Phase Margin
-40
ö_P1
ö_P3
0
-45
-90
-135
-180
-60
-225
-80
1e-1
1e1
1e3
1e5
FREQUENCY (Hz)
-270
1e7
Figure 10. Compensated Loop Gain Frequency Response
OUTPUT OVER-VOLTAGE LOCKOUT (OVLO)
V_O
R_{OV2 OVP}
R_OV1
LM3429
20 PA
1.24V
OVLO
Figure 11. Over-Voltage Protection Circuitry
The LM3429 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 11. 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 12.
16
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