Texas Instruments LM3423 Evaluation Board LM3423BBLSCSEV/NOPB LM3423BBLSCSEV/NOPB Scheda Tecnica
Codici prodotto
LM3423BBLSCSEV/NOPB
1
P
=
Z
1+D
O
D
C
r x
3
=
0
U
T
=
SNS
CSH
R
R
500V
D
x
x
x
c
620V
D
x
c
(
)
LIM
LED
R
I
D
1
x
x
+
(
)
LIM
HSP
R
R
D
1
x
x
+
x
=
¨
¨
©
©
§
+
s
1
Z
1
P
¸
¸
¹
¹
·
0
U
T
U
T
¨
¨
©
©
§
-
s
1
Z
1
Z
¸
¸
¹
¹
·
LM3421/23
V
IN
EN
nDIM
V
IN
V
O
RPD
OVP
Enable
R
UV2
R
UV1
R
OV2
R
OV1
L1
D1
SNVS574E – JULY 2008 – REVISED MAY 2013
OVER-CURRENT PROTECTION
The LM3421/23 devices have a secondary method of over-current protection. Switching action is disabled
whenever the current in the LEDs is more than 30% above the regulation set point. The dimming MosFET switch
driver (DDRV) is not disabled however as this would immediately remove the fault condition and cause oscillatory
behavior.
whenever the current in the LEDs is more than 30% above the regulation set point. The dimming MosFET switch
driver (DDRV) is not disabled however as this would immediately remove the fault condition and cause oscillatory
behavior.
ZERO CURRENT SHUTDOWN
The LM3421/23 devices implement "zero current" shutdown via the EN and RPD pins. When pulled low, the EN
pin places the devices into near-zero current state, where only the leakage currents will be observed at the pins
(typical 0.1 µA). The applications circuits, frequently have resistor dividers to set UVLO, OVLO, or other similar
functions. The RPD pin is an open drain N-channel MosFET that is enabled only when the device is enabled.
Tying the bottom of all resistor dividers to the RPD pin as shown in
pin places the devices into near-zero current state, where only the leakage currents will be observed at the pins
(typical 0.1 µA). The applications circuits, frequently have resistor dividers to set UVLO, OVLO, or other similar
functions. The RPD pin is an open drain N-channel MosFET that is enabled only when the device is enabled.
Tying the bottom of all resistor dividers to the RPD pin as shown in
allows them to float during
shutdown, thus removing their current paths and providing true application-wide zero current shutdown.
Figure 21. Zero Current Shutdown Circuit
CONTROL LOOP COMPENSATION
The LM3421/23 control loop is modeled like any current mode controller. Using a first order approximation, the
uncompensated loop can be modeled as a single pole created by the output capacitor and, in the boost and
buck-boost topologies, a right half plane zero created by the inductor, where both have a dependence on the
LED string dynamic resistance. There is also a high frequency pole in the model, however it is near the switching
frequency and plays no part in the compensation design process therefore it will be neglected. Since ceramic
capacitance is recommended for use with LED drivers due to long lifetimes and high ripple current rating, the
ESR of the output capacitor can also be neglected in the loop analysis. Finally, there is a DC gain of the
uncompensated loop which is dependent on internal controller gains and the external sensing network.
uncompensated loop can be modeled as a single pole created by the output capacitor and, in the boost and
buck-boost topologies, a right half plane zero created by the inductor, where both have a dependence on the
LED string dynamic resistance. There is also a high frequency pole in the model, however it is near the switching
frequency and plays no part in the compensation design process therefore it will be neglected. Since ceramic
capacitance is recommended for use with LED drivers due to long lifetimes and high ripple current rating, the
ESR of the output capacitor can also be neglected in the loop analysis. Finally, there is a DC gain of the
uncompensated loop which is dependent on internal controller gains and the external sensing network.
A buck-boost regulator will be used as an example case. See the
section for compensation of all
topologies.
The uncompensated loop gain for a buck-boost regulator is given by the following equation:
(15)
Where the uncompensated DC loop gain of the system is described as:
(16)
And the output pole (
ω
P1
) is approximated:
(17)
16
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