LT1111 Datasheet, PDF (10/16 Pages) Linear Technology – Micropower DC/DC Converter Adjustable and Fixed 5V, 12V

LT1111
APPLICATI S I FOR ATIO
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 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
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_IN –
V_CESAT to appear across L1. A current then builds up in L1.
L1
D1
V_IN
V_OUT
R3*
I_LIM
V _IN
SW1
LT1111 FB
R2
+
C1
GND
SW2
R1
*OPTIONAL
LT1111 • F04
Figure 4. Step-Up Mode Hookup.
Refer to Table 1 for Component Values.
At the end of the switch ON time the current in L1 is¹:
I_PEAK
=
V_{I N}
L
t_ON
(20)
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_OUT + V_D, the
inductor current flows through D1 into C1, increasing
V_OUT. This action is repeated as needed by the LT1111 to
keep V_FB at the internal reference voltage of 1.25V. R1 and
R2 set the output voltage according to the formula
( ) ^V_OUT
=
_{1 +}
R2
R1
1.25V
(21)
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.
V_IN
R3
100 Ω
+
I_LIM
C2
V_IN SW1
FB
LT1111
SW2
GND
L1
D1 +
1N5818
C1
V_OUT
R2
R1
Figure 5. Step-Down Mode Hookup
LT1111 • F05
When the switch turns on, SW2 pulls up to V_IN – V_SW. This
puts a voltage across L1 equal to V_IN – V_SW – V_OUT,
causing a current to build up in L1. At the end of the switch
ON time, the current in L1 is equal to:
I_PEAK
=
V_{IN −}
V_SW
L
−
V_OUT
t _ON
(22)
Note 1: This simple expression neglects the effect of switch and coil
resistance. This is taken into account in the “Inductor Selection” section.
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