Datasheet

13
LT3466-1
34661f
APPLICATIO S I FOR ATIO
WUUU
Figure 9 shows the feedback voltage variation versus the
control voltage. As seen in Figure 9, the linearity of the
graph allows the feedback voltage to be set accurately via
the control voltage.
The boost converter output voltage (V
OUT2
) is given by:
VV
R
R
OUT FB22
1
1
2
=+
Thus a linear change in the feedback (FB2) voltage results
in a linear change in the boost output voltage (V
OUT2
).
Connect the CTRL2 pin to ground to disable converter 2.
Do not leave the pin floating. Unlike the CTRL1 pin, which
has an internal 100k pull-down resistor, the CTRL2 pin
input impedance is very high (>100M). A small amount
of board leakage current is sufficient to turn on the
converter 2.
The R
BASE
resistor can be calculated as:
I
LOAD
= 30mA
I
I
h
BASE
LOAD
FE MIN
=
04.
()
I
BASE
must be chosen such that Q1 is in saturation under
all conditions. The h
FE(MIN)
can be obtained from the
Philips BC807 data sheet as:
h
FE(MIN)
100
This yields worst case I
BASE
as:
I
mA
mA
BASE
=
30
0 4 100
075
.( )
.
R
BASE
is given by:
VIRVVV
IN MAX BASE BASE BE Q OUT CE Q( ) () ()
+• + = +
121
Thus R
VV V V
I
BASE
OUT IN MAX CE Q BE Q
B
;
––
( ) () ()
=
+
211
AASE
Figure 10. Li-Ion Powered Driver for 6 White LEDs and a
Secondary OLED Display with Output Disconnect
CTRL1 CTRL2
R
T
SW1 SW2V
IN
3V TO 5V
16V
30mA
L2
33µH
C
IN
1µF
R
BASE
I
BASE
V
CE(SAT)
C
OUT1
1µF
C
OUT2
0.47µF
63.4k
1%
L1
33µH
LT3466-1
C
IN
: TAIYO YUDEN JMK107BJ105
C
OUT1
: TAIYO YUDEN GMK316BJ105
C
OUT2
, C
OUT3
: TAIYO YUDEN TMK316BJ474
L1, L2: TOKO D52LC
Q1: PHILIPS BC807
V
OUT1
FB1
R
FB1
10
R2
24.9k
R1
475k
34661 F10
V
OUT2
FB2
ONOFFONOFF
C
OUT3
0.47µF
Q1
+–
Figure 9. V
FB2
vs V
CTRL2
OUTPUT DISCONNECT
The LT3466-1 can be used for powering white LEDs
(Channel 1) and an OLED display or, LCD bias (Channel 2).
Some OLED displays require load isolation in order to
reduce the current drained from the battery in shutdown.
The LT3466-1 output can be configured to provide output
disconnect by the use of only one resistor, R
BASE
, and a
PNP transistor, Q1, as shown in Figure 10.
As a design example, we target a Li-Ion powered driver for
6 white LEDs and an OLED display (16V at 30mA). We can
choose a general purpose PNP switching transistor like
Philips BC807 (Q1) to provide isolation.
V
CTRL2
(V)
0
V
FB2
(mV)
400
600
2
34661 F09
200
300
500
100
0
0.4
0.8
1.61.2
900
800
700
V
IN
= 3.6V
V
OUT2
= 16V