Datasheet

LT8410/LT8410-1
9
84101fb
or 0805 size capacitor will be adequate. A 0.1μF to 1μF
capacitor placed on the CAP node is recommended to filter
the inductor current, while a 0.1μF to 1μF capacitor placed
on the V
OUT
node will give excellent transient response
and stability. To make the V
REF
pin less sensitive to noise,
putting a capacitor on the V
REF
pin is recommended, but not
required. A 47nF to 220nF 0402 capacitor will be sufficient.
Table 2 shows a list of several capacitor manufacturers.
Consult the manufacturers for more detailed information
and for their entire selection of related parts.
Table 2. Recommended Ceramic Capacitor Manufacturers
MANUFACTURER PHONE WEB SITE
Taiyo Yuden [phone] www.t-yuden.com
Murata [phone] www.murata.com
AVX [phone] www.avxcorp.com
Kemet [phone] www.kemet.com
TDK [phone] www.tdk.com
Setting Output Voltage
The output voltage is set by the FBP pin voltage. V
OUT
is
equal to 31.85 • V
FBP
when the output is regulated, as
shown in Figure 1. Since the V
REF
pin provides a good
reference (1.235V), the FBP voltage can be easily set by
a resistor divider from the V
REF
pin to ground. The series
resistance of this resistor divider should be kept larger than
200KΩ to prevent loading down the V
REF
pin. The FBP pin
can also be biased directly by an external reference. For
overvoltage protection, the output voltage is limited to
40V. Therefore, if V
FBP
is higher than 1.235V, the output
voltage will stay at 40V.
APPLICATIONS INFORMATION
FBP VOLTAGE (V)
0
OUTPUT VOLTAGE (V)
50
40
30
10
20
0
8410-1 F01
21 1.50.5
Figure 1. FBP to V
OUT
Transfer Curve
Connecting the Load to the CAP Node
The efficiency of the converter can be improved by con-
necting the load to the CAP pin instead of the V
OUT
pin.
The power loss in the PMOS disconnect circuit is then
made negligible. No quiescent current will be consumed
in the internal feedback resistor divider string during
shutdown since the PMOS transistor will be open and the
internal feedback resistor divider is connected at the V
OUT
pin. The disadvantage of this method is that the CAP node
cannot go to ground during shutdown, but will be limited
to around a diode drop below V
CC
. Loads connected to the
part should only sink current. Never force external power
supplies onto the CAP or V
OUT
pins.
Maximum Output Load Current
The maximum output current of a particular LT8410 series
circuit is a function of several circuit variables. The following
method can be helpful in predicting the maximum load
current for a given circuit:
Step 1. Calculate the peak inductor current:
I
PK
= I
LIMIT
+
V
IN
•150 • 10
–6
L
mA
where I
LIMIT
is 25mA and 8mA for LT8410 and LT8410-1
respectively. L is the inductance value in henrys and V
IN
is the input voltage to the boost circuit.
Step 2. Calculate the inductor ripple current:
I
RIPPLE
=
V
OUT
+ 1– V
IN
( )
• 200 • 10
–6
L
mA
where V
OUT
is the desired output voltage. If the inductor
ripple current is less than the peak current, then the circuit
will only operate in discontinuous conduction mode. The
inductor value should be increased so that I
RIPPLE
< I
PK
.
An application circuit can be designed to operate only in
discontinuous mode, but the output current capability
will be reduced.
Step 3. Calculate the average input current:
I
IN(AVG)
= I
PK
–
I
RIPPLE
2
mA