LM3500_14 Datasheet, PDF (15/25 Pages) Texas Instruments – Synchronous Step-up DC/DC Converter for White LED Applications

LM3500
www.ti.com
SNVS231F – MAY 2004 – REVISED APRIL 2007
The LM3500 has an internal thermal shutdown function to protect the die from excessive temperatures. The
thermal shutdown trip point is typically 150°C. There is a hysteresis of typically 35°C so the die temperature must
decrease to approximately 115°C before the LM3500 will return to normal operation.
INDUCTOR SELECTION
The inductor used with the LM3500 must have a saturation current greater than the cycle by cycle peak inductor
current (see Typical Peak Inductor Currents table below). Choosing inductors with low DCR decreases power
losses and increases efficiency.
The minimum inductor value required for the LM3500-16 can be calculated using the following equation:
V_IN R_DSON
L>
D _-1
0.29 D'
(2)
The minimum inductor value required for the LM3500-21 can be calculated using the following equation:
V_IN R_DSON
L>
D _-1
0.58 D'
(3)
For both equations above, L is in µH, V_IN is the input supply of the chip in Volts, R_DSON is the ON resistance of
the NMOS power switch found in the Typical Performance Characteristics section in ohms and D is the duty
cycle of the switching regulator. The above equation is only valid for D greater than or equal to 0.5. For
applications where the minimum duty cycle is less than 0.5, a 22µH inductor is the typical recommendation for
use with most applications. Bench-level verification of circuit performance is required in these special cases,
however. The duty cycle, D, is given by the following equation:
V_IN
D' = _VOUT =1-D
(4)
where V_OUT is the voltage at pin C1.
Table 1. Typical Peak Inductor Currents (mA)
# LEDs
LED Current
V_IN
(V)
(in
series)
15
mA
20
mA
30
mA
40
mA
50
mA
60
mA
2.7
2
82
100
134
160
204
234
3
118
138
190
244
294
352
4
142
174
244
322
X
X
5
191
232
319
413
X
X
3.3
2
76
90
116
136
172
198
3
110
126
168
210
250
290
4
132
158
212
270
320
X
5
183
216
288
365
446
X
4.2
2
64
76
96
116
142
162
3
102
116
148
180
210
246
4
122
146
186
232
272
318
5
179
206
263
324
388
456
The typical cycle-by-cycle peak inductor current can be calculated from the following equation:
| IPK
I_OUT V_IND
+
KD' 2LF_SW
(5)
where I_OUT is the total load current, F_SW is the switching frequency, L is the inductance and η is the converter
efficiency of the total driven load. A good typical number to use for η is 0.8. The value of η can vary with load and
duty cycle. The average inductor current, which is also the average V_SW pin current, is given by the following
equation:
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