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LT1111IS8

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P16
10
LT1111
1111fd
U
S
A
O
PP
L
IC
AT
I
WU
U
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.
At the end of the switch ON time the current in L1 is
1
:
I
V
L
t
PEAK
IN
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
R
R
V
OUT
=+
()
1
2
1
125 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.
L1
LT1111 • F04
GND SW2
SW1
LIM
I
IN
V
D1
R3*
LT1111
V
OUT
R2
R1
C1
*OPTIONAL
V
IN
FB
+
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.
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
V
VV
L
t
PEAK
IN
SW OUT
ON
=
−−
()22
LT1111 • F05
GND
SW2
SW1
LIM
I
IN
V
R3
100
FB
V
OUT
C2
C1
D1
1N5818
V
IN
R2
R1
L1
Ω
LT1111
+
+
Figure 5. Step-Down Mode Hookup
11
L T 1111
1111fd
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Figure 6. Q1 Permits Higher Current Switching.
LT1111 Functions as Controller.
When the switch turns off, the SW2 pin falls rapidly and
actually goes below ground. D1 turns on when SW2
reaches 0.4V below ground.
D1 MUST BE A SCHOTTKY
DIODE
. The voltage at SW2 must never be allowed to go
below –0.5V. A silicon diode such as the 1N4933 will allow
SW2 to go to –0.8V, causing potentially destructive power
dissipation inside the LT1111. Output voltage is deter-
mined by:
V
R
R
V
OUT
=+
()
1
2
1
125 23.()
R3 programs switch current limit. This is especially impor-
tant in applications where the input varies over a wide
range. Without R3, the switch stays on for a fixed time each
cycle. Under certain conditions the current in L1 can build
up to excessive levels, exceeding the switch rating and/or
saturating the inductor. The 100Ω resistor programs the
switch to turn off when the current reaches approximately
700mA. When using the LT1111 in step-down mode,
output voltage should be limited to 6.2V or less. Higher
output voltages can be accommodated by inserting a
1N5818 diode in series with the SW2 pin (anode con-
nected to SW2).
Higher Current Step-Down Operation
Output current can be increased by using a discrete PNP
pass transistor as shown in Figure 6. R1 serves as a
current limit sense. When the voltage drop across R1
equals a V
BE
, the switch turns off. For temperature com-
pensation a Schottky diode can be inserted in series with
the I
LIM
pin. This also lowers the maximum drop across R1
to V
BE
– V
D
, increasing efficiency. As shown, switch
current is limited to 2A. Inductor value can be calculated
based on formulas in the “Inductor Selection — Step-
Down Converter” section with the following conservative
expression for V
SW
:
VVV V
SW R Q SAT
=+ ≈
11
10 24.()
R2 provides a current path to turn off Q1. R3 provides base
drive to Q1. R4 and R5 set output voltage. A PMOS FET can
be used in place of Q1 when V
IN
is between 10V and 20V.
LT1111 • TA08
D1
1N5821
V
OUT
V
IN
30V
MAX
L1
R1
0.3Ω
R2
220
Q1
MJE210 OR
ZETEX ZTX749
R3
330
R4
R5
C1
V
OUT
= 1.25V (1 + )
R4
R5
C2
+ +
LT1111
GND
SW2
SW1
V
IN
I
L
FB
In Figure 8, the input is negative while the output is
positive. In this configuration, the magnitude of the input
voltage can be higher or lower than the output voltage. A
level shift, provided by the PNP transistor, supplies proper
polarity feedback information to the regulator.
Inverting Configurations
The LT1111 can be configured as a positive-to-negative
converter (Figure 7), or a negative-to-positive converter
(Figure 8). In Figure 7, the arrangement is very similar to
a step-down, except that the high side of the feedback is
referred to ground. This level shifts the output negative. As
in the step-down mode, D1 must be a Schottky diode,
and V
OUT
should be less than 6.2V. More negative out-
put voltages can be accommodated as in the prior section.
Figure 7. Positive-to-Negative Converter
12
LT1111
1111fd
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L1
LT1111 • F08
GND SW2
FB
SW1
LIM
I
IN
V
D1
A0
V
OUT
R2
V
OUT
= 1.25V + 0.6V
( )
R1
R2
R1
2N3906
–V
IN
C1
LT1111
C2
+
+
Figure 8. Negative-to-Positive Converter
Using the I
LIM
Pin
The LT1111 switch can be programmed to turn off at a set
switch current, a feature not found on competing devices.
This enables the input to vary over a wide range without
exceeding the maximum switch rating or saturating the
inductor. Consider the case wh ere analysis shows the
LT1111 must operate at an 800mA peak switch current
with a 2V input. If V
IN
rises to 4V, the peak switch current
will rise to 1.6A, exceeding the maximum switch current
rating. With the proper resistor selected (see the “Maxi-
mum Switch
Current vs I
LIM
” characteristic), the switch
current will be limited to 800mA, even if the input voltage
increases.
Another situation where the I
LIM
feature is useful occurs
when the device goes into continuous mode operation.
This occurs in step-up mode when:
V
V
VV DC
OUT DIODE
IN SW
+
−
<
−
1
1
25()
When the input and output voltages satisfy this relation-
ship, inductor current does not go to zero during the
switch OFF time. When the switch turns on again, the
current ramp starts from the non-zero current level in the
inductor just prior to switch turn-on. As shown in Figure
9, the inductor current increases to a high level before the
comparator turns off the oscillator. This high current can
cause excessive output ripple and requires oversizing the
output capacitor and inductor. With the I
LIM
feature,
however, the switch current turns off at a programmed
level as shown in Figure 10, keeping output ripple to a
minimum.
LT1111 • F09
I
OFF
L
ON
SWITCH
Figure 9. No Current Limit Causes Large Inductor
Current Build-Up
LT1111 • F10
I
ON
L
OFF
SWITCH
PROGRAMMED CURRENT LIMIT
Figure 10. Current Limit Keeps Inductor Current Under Control
Figure 11 details current limit circuitry. Sense transistor
Q1, whose base and emitter are paralleled with power
switch Q2, is ratioed such that approximately 0.5% of Q2’s
collector current flows in Q1’s collector. This current is
passed through internal 80Ω resistor R1 and out through
the I
LIM
pin. The value of the external resistor connected
between I
LIM
and V
IN
sets the current limit. When suffi-
cient switch current flows to develop a V
BE
across R1 +
R
LIM
, Q3 turns on and injects current into the oscillator,
turning off the switch. Delay through this circuitry is
approximately 1µs. The current trip point becomes less
accurate for switch ON times less than 3µs. Resistor
values programming switch ON time for 1µs or less will
cause spurious response in the switch circuitry although
the device will still maintain output regulation.
LT1111 • F11
SW2
SW1
DRIVER
OSCILLATOR
V
IN
I
LIM
R1
80Ω
(INTERNAL)
R
LIM
(EXTERNAL)
Q1
Q2
Q3
Figure 11. LT1111 Current Limit Circuitry
P1-P3P4-P6P7-P9P10-P12P13-P15P16-P16

LT1111IS8

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Manufacturer:
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Description:
Switching Voltage Regulators LT1111 - Micropower DC/DC Converter Adjustable and Fixed 5V, 12V
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