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LT3466-1
APPLICATIO S I FOR ATIO
Figure 9 shows the feedback voltage variation versus the
control voltage. As seen in Figure 9, the linearity of the
3V TO 5V
= V FB 2 ? 1 +
?
? R 1 ?
?
R 2 ?
graphallowsthefeedbackvoltagetobesetaccuratelyvia
the control voltage.
The boost converter output voltage (V OUT2 ) is given by:
V OUT 2
C OUT1
1 μ F
L1
33 μ H
SW1
V OUT1
C IN
1 μ F
L2
33 μ H
V IN SW2
V OUT2
LT3466-1
I BASE R BASE
Q1
+ –
V CE(SAT)
R1
475k
C OUT2
0.47 μ F
16V
30mA
C OUT3
0.47 μ F
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.
R FB1
10 ?
OFF ON
FB1
CTRL1
R T
FB2
CTRL2
63.4k
1%
OFF ON
R2
24.9k
34661 F10
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.
900
V IN = 3.6V
800 V OUT2 = 16V
700
600
C IN : TAIYO YUDEN JMK107BJ105
C OUT1 : TAIYO YUDEN GMK316BJ105
C OUT2 , C OUT3 : TAIYO YUDEN TMK316BJ474
L1, L2: TOKO D52LC
Q1: PHILIPS BC807
Figure 10. Li-Ion Powered Driver for 6 White LEDs and a
Secondary OLED Display with Output Disconnect
The R BASE resistor can be calculated as:
I LOAD = 30mA
500
400
300
I BASE =
I LOAD
0 . 4 h FE ( MIN )
200
100
I BASE must be chosen such that Q1 is in saturation under
all conditions. The h FE(MIN) can be obtained from the
0
0
0.4
0.8 1.2
V CTRL2 (V)
1.6
2
Philips BC807 data sheet as:
h FE(MIN) ? 100
Figure 9. V FB2 vs V CTRL2
34661 F09
This yields worst case I BASE as:
OUTPUT DISCONNECT
The LT3466-1 can be used for powering white LEDs
I BASE =
30 mA
0 . 4 ( 100 )
? 0 . 75 mA
(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.
R BASE is given by:
V IN ( MAX ) + I BASE ? R BASE + V BE ( Q 1 ) = V OUT 2 + V CE ( Q 1 )
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.
Thus ; R BASE =
V OUT 2 – V IN ( MAX ) + V CE ( Q 1 ) – V BE ( Q 1 )
I B A SE
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.
34661f
13
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