LT1111CS85PBF LINEAR TECH, LT1111CS85PBF Datasheet - Page 10

• Manufacturer Part Number: LT1111CS85PBF
• Manufacturer: LINEAR TECH

LT1111
Diode Selection
Speed, forward drop, and leakage current are the three
main considerations in selecting a catch diode for LT1111
converters. General purpose rectifiers such as the 1N4001
are unsuitable for use in any switching regulator applica-
tion. Although they are rated at 1A, the switching time of
a 1N4001 is in the 10 s to 50 s range. At best, efficiency
will be severely compromised when these diodes are
used; at worst, the circuit may not work at all. Most
LT1111 circuits will be well served by a 1N5818 Schottky
diode, or its surface mount equivalent, the MBRS130T3.
The combination of 500mV forward drop at 1A current,
fast turn ON and turn OFF time, and 4 A to 10 A leakage
current fit nicely with LT1111 requirements. At peak
switch currents of 100mA or less, a 1N4148 signal diode
may be used. This diode has leakage current in the 1nA to
5nA range at 25 C and lower cost than a 1N5818. (You can
also use them to get your circuit up and running, but
beware of destroying the diode at 1A switch currents.)
Step-Up (Boost Mode) Operation
A step-up DC/DC converter delivers an output voltage
higher than the input voltage. Step-up converters are not
short-circuit protected since there is a DC path from input
to output.
The usual step-up configuration for the LT1111 is shown
in Figure 4. The LT1111 first pulls SW1 low causing V
V
10
A
V
*OPTIONAL
IN
CESAT
PPLICATI
to appear across L1. A current then builds up in L1.
GND
I
LIM
Figure 4. Step-Up Mode Hookup.
Refer to Table 1 for Component Values.
R3*
LT1111
O
V
SW2
IN
SW1
U
FB
S
L1
I FOR ATIO
U
D1
W
R2
R1
+
C1
LT1111 • F04
U
V
OUT
IN
–
At the end of the switch ON time the current in L1 is
Immediately after switch turn-off, the SW1 voltage pin
starts to rise because current cannot instantaneously stop
flowing in L1. When the voltage reaches V
inductor current flows through D1 into C1, increasing
V
keep V
R2 set the output voltage according to the formula
Step-Down (Buck Mode) Operation
A step-down DC/DC converter converts a higher voltage
to a lower voltage. The usual hookup for an LT1111 based
step-down converter is shown in Figure 5.
When the switch turns on, SW2 pulls up to V
puts a voltage across L1 equal to V
causing a current to build up in L1. At the end of the switch
ON time, the current in L1 is equal to:
Note 1: This simple expression neglects the effect of switch and coil
resistance. This is taken into account in the “Inductor Selection” section.
V
OUT
IN
I
I
V
PEAK
PEAK
OUT
. This action is repeated as needed by the LT1111 to
+
FB
at the internal reference voltage of 1.25V. R1 and
C2
V
V
1
L
I
I N
IN
LIM
Figure 5. Step-Down Mode Hookup
R3
100
t
R
ON
R
LT1111
GND
V
2
1
V
SW
IN
L
1 25
SW2
SW1
.
FB
V
OUT
V
t
ON
D1
1N5818
L1
+
IN
– V
C1
OUT
IN
SW
– V
+ V
R2
R1
LT1111 • F05
SW
– V
D
. This
V
, the
1
OUT
OUT
1111fd
( )
( )
:
( )
20
22
21
,