Texas Instruments LM3429 Evaluation Boards LM3429BSTEVAL/NOPB LM3429BSTEVAL/NOPB Datenbogen
Produktcode
LM3429BSTEVAL/NOPB
=
D
r
2
D
c
x
1
Z
Z
L1
D x
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
·
I
LIM
=
245 mV
R
LIM
SNVS616G – APRIL 2009 – REVISED MAY 2013
There are two possible methods to sense the transistor current. The R
DS-ON
of the main power MosFET can be
used as the current sense resistance because the IS pin was designed to withstand the high voltages present on
the drain when the MosFET is in the off state. Alternatively, a sense resistor located in the source of the MosFET
may be used for current sensing, however a low inductance (ESL) type is suggested. The cycle-by-cycle current
limit (I
the drain when the MosFET is in the off state. Alternatively, a sense resistor located in the source of the MosFET
may be used for current sensing, however a low inductance (ESL) type is suggested. The cycle-by-cycle current
limit (I
LIM
) can be calulated using either method as the limiting resistance (R
LIM
):
(12)
In general, the external series resistor allows for more design flexibility, however it is important to ensure all of
the noise sensitive low power ground connections are connected together local to the controller and a single
connection is made to the high current PGND (sense resistor ground point).
the noise sensitive low power ground connections are connected together local to the controller and a single
connection is made to the high current PGND (sense resistor ground point).
CONTROL LOOP COMPENSATION
The LM3429 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 above 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 above 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:
(13)
Where the uncompensated DC loop gain of the system is described as:
(14)
And the output pole (
ω
P1
) is approximated:
(15)
And the right half plane zero (
ω
Z1
) is:
(16)
14
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