ADM202 Data Sheet
Rev. B | Page 8 of 10
APPLICATIONS INFORMATION
The internal circuitry consists of three main sections, as follows:
• A charge pump dc-to-dc voltage converter
• TTL/CMOS to RS-232 transmitters
• RS-232 to TTL/CMOS receivers
CHARGE PUMP DC-TO-DC VOLTAGE CONVERTER
The charge pump voltage converter consists of an oscillator and
a switching matrix. The converter generates a ±10 V supply from
the input 5 V level in two stages, using a switched capacitor
technique as illustrated in Figure 9 and Figure 10. First, the 5 V
input supply is doubled to 10 V using Capacitor C1 as the
charge storage element. The 10 V level is then inverted to
generate −10 V using C2 as the storage element.
Capacitors C3 and C4 reduce the output ripple. Their values are
not critical and can be reduced if higher levels of ripple are
acceptable. The charge pump capacitors, C1 and C2, can also be
reduced at the expense of higher output impedance on the V+
and V− supplies.
The V+ and V− supplies can be used to power external circuitry
if the current requirements are small.
+ +
S3S1
V+ = 2V
CC
S4S2
INTERNAL
OSCILLATOR
C3
C1
V
CC
GND
V
CC
00065-009
Figure 9. Charge Pump Voltage Doubler
+ +
S3
GND
S4S2
S1
INTERNAL
OSCILLATOR
C4
C2
V+
GND
V– = –(V+)
FROM
VOLTAGE
DOUBLER
00065-010
Figure 10. Charge Pump Voltage Inverter
TTL/CMOS TO RS-232 TRANSMITTERS (DRIVERS)
The drivers convert TTL/CMOS input levels into TIA/EIA-232-E
output levels. With V
CC
= 5 V and driving a typical TIA/EIA-232-E
load, the output voltage swing is ±9 V. Even under worst case
conditions, the drivers are guaranteed to meet the ±5 V TIA/EIA-
232-E minimum requirement.
The input threshold levels are both TTL and CMOS compatible
with the switching threshold set at V
CC
/4. With a nominal V
CC
=
5 V, the switching threshold is 1.25 V typical. Unused inputs can
be left unconnected, as an internal 400 kΩ pull-up resistor pulls
them high, forcing the outputs into a low state.
As required by the TIA/EIA-232-E standard, the slew rate is
limited to less than 30 V/µs without the need for an external
slew limiting capacitor, and the output impedance in the power-
down state is greater than 300 Ω.
RS-232 TO TTL/CMOS RECEIVERS
The receivers are inverting level shifters that accept TIA/EIA-232-E
input levels (±5 V to ±15 V) and translate them into 5 V TTL/
CMOS levels. The inputs have internal 5 kΩ pull-down resistors
to ground and are protected against overvoltages of up to ±30 V.
The guaranteed switching thresholds are 0.8 V (V
INL
) and 2.4 V
(V
INH
), which are well within the ±3 V TIA/EIA-232-E require-
ment. The low level threshold is deliberately positive because it
ensures that an unconnected input is interpreted as a low level.
The receivers have a Schmitt triggered input with a hysteresis
level of 0.5 V. This hysteresis level ensures error free reception,
both for noisy inputs and for inputs with slow transition times.
