LT1111
O U
W
U
PPLICATI
A
S I FOR ATIO
Diode Selection
At the end of the switch ON time the current in L1 is1:
Speed, forward drop, and leakage current are the three
main considerations in selecting a catch diode for LT1111
converters.Generalpurposerectifierssuchasthe1N4001
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 10µs to 50µs 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 4µA to 10µA 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
5nArangeat25°Candlowercostthana1N5818. (Youcan
also use them to get your circuit up and running, but
beware of destroying the diode at 1A switch currents.)
V
IN
I
=
t
(20)
PEAK
ON
L
Immediately after switch turn-off, the SW1 voltage pin
starts to rise because current cannot instantaneously stop
flowing in L1. When the voltage reaches VOUT + VD, the
inductor current flows through D1 into C1, increasing
OUT. This action is repeated as needed by the LT1111 to
keep VFB at the internal reference voltage of 1.25V. R1 and
R2 set the output voltage according to the formula
V
R2
V
= 1+
1.25V
(
(21)
)
OUT
R1
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.
Step-Up (Boost Mode) Operation
V
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.
IN
R3
100Ω
+
I
V
IN
SW1
FB
LIM
C2
The usual step-up configuration for the LT1111 is shown
in Figure 4. The LT1111 first pulls SW1 low causing VIN –
VCESAT to appearacrossL1. Acurrentthenbuildsup inL1.
LT1111
GND
L1
V
SW2
OUT
R2
R1
D1
1N5818
+
C1
D1
L1
V
V
IN
OUT
R3*
LT1111 • F05
R2
R1
I
V
IN
SW1
LIM
Figure 5. Step-Down Mode Hookup
+
C1
LT1111 FB
When the switch turns on, SW2 pulls up to VIN – VSW. This
puts a voltage across L1 equal to VIN – VSW – VOUT
causing a current to build up in L1. At the end of the switch
ON time, the current in L1 is equal to:
,
GND
SW2
*OPTIONAL
LT1111 • F04
V
− V − V
IN
SW
OUT
I
=
t
(22)
PEAK
ON
L
Figure 4. Step-Up Mode Hookup.
Refer to Table 1 for Component Values.
Note 1: This simple expression neglects the effect of switch and coil
resistance. This is taken into account in the “Inductor Selection” section.
10
Linear [ Linear ]
