LT1111
O U
W
U
PPLICATI
A
S I FOR ATIO
Q1
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. Asilicondiodesuchasthe1N4933willallow
SW2togoto–0.8V, causingpotentiallydestructivepower
dissipation inside the LT1111. Output voltage is deter-
mined by:
R1
MJE210 OR
ZETEX ZTX749
0.3Ω
L1
V
30V
MAX
IN
V
OUT
R2
220
D1
1N5821
R3
330
V
I
L
SW1
IN
+
+
C2
C1
LT1111
R4
FB
SW2
GND
R5
R4
R2
R1
V
= 1.25V
(
1 +
)
OUT
R5
VOUT = 1+
1.25V
(23)
(
)
LT1111 • TA08
Figure 6. Q1 Permits Higher Current Switching.
LT1111 Functions as Controller.
R3programsswitchcurrentlimit.Thisisespeciallyimpor-
tant in applications where the input varies over a wide
range.WithoutR3,theswitchstaysonforafixedtimeeach
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).
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
referredtoground.Thislevelshiftstheoutputnegative.As
in the step-down mode, D1 must be a Schottky diode,
and VOUT should be less than 6.2V. More negative out-
put voltages can be accommodated as in the prior section.
V
IN
Higher Current Step-Down Operation
+
R3
C2
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 VBE, the switch turns off. For temperature com-
pensation a Schottky diode can be inserted in series with
theILIM pin.ThisalsolowersthemaximumdropacrossR1
to VBE – VD, 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
I
V
IN
SW1
FB
LIM
LT1111
GND
L1
SW2
R1
R2
D1
1N5818
+
C1
–V
OUT
LT1111 • F07
Figure 7. Positive-to-Negative Converter
expression for VSW
:
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
levelshift, providedbythePNPtransistor, suppliesproper
polarity feedback information to the regulator.
VSW = VR1 + VQ1SAT ≈ 1.0V
(24)
R2providesacurrentpathtoturnoffQ1. R3providesbase
drivetoQ1. R4andR5setoutputvoltage. APMOSFETcan
be used in place of Q1 when VIN is between 10V and 20V.
1111fd
11
Linear [ Linear ]
