LT1332_15 Datasheet, PDF (7/8 Pages) Linear Technology – Wide Supply Range Low Power RS232 Transceiver with 12V VPP Output for Flash Memory

LT1332
APPLICATIONS INFORMATION
Operation with a Switching Regulator
The LT1332 is designed to be powered from an external
switching regulator which may be used elsewhere for power
conditioning. In a typical application, the LT1332 shares
the regulator’s positive output, while charge is capacitively
pumped from the regulator’s switch pin to the negative
supply. Schottky rectifiers built into the LT1332 simplify
the charge pump design. When used with a micropower
switcher like the LT1109A, the Burst Mode^™ operation of
the charge pump resembles the switching characteristics
of the LT1237 and similar devices.
The V ^– supply is not directly regulated. The circuit relies on
cross regulation and the regulator’s minimum duty cycle
to control V^–. Select the C^–­ and V^– storage capacitors so
that when the regulator operates at minimum duty cycle,
sufficient charge will transfer to the V^– storage cap to
maintain a voltage of at least –6.5V.
While only 0.1µF ceramic decoupling capacitors are
needed on the positive supply inputs, low ESR tantalum
capacitors should be used in the charge pump to reduce
voltage losses. The C ^– capacitor should be at least 1µF
and the V^– capacitor should be 5 to 10 times bigger. As
a rule of thumb, make the V^– capacitor at least 1/DC_MIN
times bigger than the C^– capacitor where DC_MIN is the
regulator’s minimum duty cycle. Using large values for
the V^– capacitor reduces ripple on the V^– supply.
Multiple Transceivers
The circuit in Figure 1 demonstrates how the LT1332 may
be used with different types of switching regulators. Four
LT1332s are powered from a single PWM DC/DC converter
using an LT1172. Even with all twelve drivers heavily
loaded (R_L = 3k, C_L = 2500pF), the circuit generates fully
compliant RS232 signals at 120k baud.
Operations with External Supplies
When external RS232 supplies are available (6.5V ≤ V ⁺
≤ 13.2V, –13.2V ≤ V^– ≤ –6.2V) the LT1332 can be
used as a stand-alone unit. Capacitor selection is
1µF
0.1µF
V⁺
V^–
LT1332
C^–
GND
1µF
10µF 0.1µF
V⁺
V^–
LT1332
C^–
GND
1µF
10µF 0.1µF
V⁺
V^–
LT1332
C^–
GND
1µF
10µF 0.1µF
3V V_CC
INPUT
ON/OFF
0.1µF
ON/OFF
3V V_CC GND
2.7V < V_IN < 5.5V
I(V_IN) < 14mA (UNLOADED)
V_IN
68µF
V_IN
L1*
100µH
V_SW
LT1172
FB
GND
V_C
0.1µF
ON/OFF
3V V_CC GND
0.1µF
ON/OFF
3V V_CC GND
0.1µF
1N5817 8V
+
22µF
135k**
25k**
*L1 = SUMIDA CD105-101K OR COILCRAFT DO3316-104
**1% METAL FILM
V⁺
V^–
LT1332
C^–
GND
ON/OFF
3V V_CC GND
10µF
OPTIONAL
SHUTDOWN
VN2222LL
1k
0.1µF
LT1332 • F01
Figure 1. Multiple LT1332s Powered from a Single LT1172 DC/DC Converter
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no representa-
tion that the interconnection of its circuits as described herein will not infringe on existing patent rights.
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