LM3424 Datasheet, PDF (33/69 Pages) National Semiconductor (TI) – Constant Current N-Channel Controller with Thermal Foldback for Driving LEDs

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LM3424, LM3424-Q1
SNVS603C – AUGUST 2009 – REVISED AUGUST 2015
Typical Applications (continued)
If the calculated R_HSP is too far from a desired standard value, then R_CSH can be adjusted to obtain a standard
value.
8.2.1.2.4 Thermal Foldback
For all topologies, set the thermal foldback breakpoint (T_BK) by finding corresponding R_NTC-BK from manufacturer's
datasheet and solving for R_BIAS:
R_BIAS
=
R_NTC-BK
x
R_REF2
R_REF1
(44)
The easiest approach is to set R_REF1 = R_REF2, therefore setting R_BIAS = R_NTC-BK will properly set T_BK. Remember,
capacitance is recommended at the TSENSE and TREF pins, so ensure C_REF > C_NTC to prevent start-up in
foldback.
Then set the thermal foldback endpoint (T_END) by finding the corresponding R_NTC-END from manufacturer's
datasheet and solving for R_GAIN:
R_GAIN
=
¨¨©§_RREF^R_{1 +}^RE_R^F1REF2
-
R_{NTC - END}
R_{NTC -END} + R_BIAS
I_CSH
¸¸¹·
x
2.45V
(45)
8.2.1.2.5 Inductor Ripple Current
Set the nominal inductor ripple current (Δi_L-PP) by solving for the appropriate inductor (L1):
Buck
L1
=
(V_IN - V_O) x D
üi
L - PP
x
f_SW
(46)
Boost and Buck-boost
L1=_üiVL-IPNP^xxDf_SW
(47)
To set the worst case inductor ripple current, use V_IN-MAX and D_MIN when solving for L1.
The minimum allowable inductor RMS current rating (I_L-RMS) can be calculated as:
Buck
¨©§ ¸¹· ^{IL-RMS = ILED x}
1 + ¹ x ^{'IL-PP 2}
12 ILED
(48)
Boost and Buck-Boost
§ · ^ILED
¨© ¸¹ ^IL-RMS ⁼ _D' ^x
1+ ¹ x
12
'I_L-PP x D'
I_LED
2
(49)
8.2.1.2.6 LED Ripple Current
Set the nominal LED ripple current (Δi_LED-PP), by solving for the output capacitance (C_O):
Buck
C_O =
8 x f_SW
i_{L - PP}
x r_D x i_{LED- PP}
(50)
Boost and Buck-Boost
C_O
=
r_D
I_LED x D
xüi_{LED- PP}
x
f_SW
(51)
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