ISL4270E Datasheet, PDF (6/13 Pages) Intersil Corporation – QFN Packaged, ±15kV ESD Protected, +3V to +5.5V, 300nA, 250kbps, RS-232 Transceivers with Enhanced Automatic Powerdown and a Separate Logic Supply

ISL4270E
Detailed Description
The ISL4270E operates from a single +3V to +5.5V supply,
guarantees a 250kbps minimum data rate, requires only four
small external 0.1μF capacitors, features low power
consumption, and meets all ElA RS-232C and V.28
specifications. The circuit is divided into three sections: The
charge pump, the transmitters, and the receivers.
Charge-Pump
Intersil’s new ISL4270E utilizes regulated on-chip dual
charge pumps as voltage doublers, and voltage inverters to
generate ±5.5V transmitter supplies from a V_CC supply as
low as 3.0V. This allows these devices to maintain RS-232
compliant output levels over the ±10% tolerance range of
3.3V powered systems. The efficient on-chip power supplies
require only four small, external 0.1μF capacitors for the
voltage doubler and inverter functions over the full V_CC
range; other capacitor combinations can be used as shown
in Table 3. The charge pumps operate discontinuously (i.e.,
they turn off as soon as the V+ and V- supplies are pumped
up to the nominal values), resulting in significant power
savings.
Transmitters
The transmitters are proprietary, low dropout, inverting
drivers that translate TTL/CMOS inputs to EIA/TIA-232
output levels. Coupled with the on-chip ±5.5V supplies,
these transmitters deliver true RS-232 levels over a wide
range of single supply system voltages.
All transmitter outputs disable and assume a high
impedance state when the device enters the powerdown
mode (see Table 2). These outputs may be driven to ±12V
when disabled.
All devices guarantee a 250kbps data rate for full load
conditions (3kΩ and 1000pF), V_CC ≥ 3.0V, with one
transmitter operating at full speed. Under more typical
conditions of V_CC ≥ 3.3V, R_L = 3kΩ, and C_L = 250pF, one
transmitter easily operates at 1.25Mbps.
The transmitter input threshold is set by the voltage applied
to the V_L supply pin. Transmitter inputs float if left
unconnected (there are no pull-up resistors), and may cause
I_CC increases. Connect unused inputs to GND for the best
performance.
Receivers
The ISL4270E contains standard inverting receivers that
convert RS-232 signals to CMOS output levels and accept
inputs up to ±25V while presenting the required 3kΩ to 7kΩ
input impedance (see Figure 1) even if the power is off
(V_CC = 0V). The receivers’ Schmitt trigger input stage uses
hysteresis to increase noise immunity and decrease errors
due to slow input signal transitions. Receiver outputs swing
from GND to V_L, and do not tristate in powerdown (see
Table 2).
V_L
R_XIN
-25V ≤ V_RIN ≤ +25V
5kΩ
GND
R_XOUT
GND ≤ V_ROUT ≤ V_L
FIGURE 1. RECEIVER CONNECTIONS
Low Power Operation
This 3V device requires a nominal supply current of 0.3mA,
even at V_CC = 5.5V, during normal operation (not in
powerdown mode). This is considerably less than the 11mA
current required by comparable 5V RS-232 devices, allowing
users to reduce system power simply by replacing the old
style device with the ISL4270E in new designs.
Powerdown Functionality
The already low current requirement drops significantly
when the device enters powerdown mode. In powerdown,
supply current drops to 1μA, because the on-chip charge
pump turns off (V+ collapses to V_CC, V- collapses to GND),
and the transmitter outputs tristate. This micro-power mode
makes these devices ideal for battery powered and portable
applications.
Software Controlled (Manual) Powerdown
This device allows the user to force the IC into the low
power, standby state, and utilizes a two pin approach where
the FORCEON and FORCEOFF inputs determine the IC’s
mode. For always enabled operation, FORCEON and
FORCEOFF are both strapped high. To switch between
active and powerdown modes, under logic or software
control, only the FORCEOFF input need be driven. The
FORCEON state isn’t critical, as FORCEOFF dominates
over FORCEON. Nevertheless, if strictly manual control over
powerdown is desired, the user must strap FORCEON high
to disable the enhanced automatic powerdown circuitry.
Connecting FORCEOFF and FORCEON together disables
the enhanced automatic powerdown feature, enabling them
to function as a manual SHUTDOWN input (see Figure 2).
With any of the above control schemes, the time required to
exit powerdown, and resume transmission is only 100μs.
When using both manual and enhanced automatic
powerdown (FORCEON = 0), the ISL4270E won’t power up
from manual powerdown until both FORCEOFF and
FORCEON are driven high, or until a transition occurs on a
receiver or transmitter input. Figure 3 illustrates a circuit for
ensuring that the ISL4270E powers up as soon as
FORCEOFF switches high. The rising edge of the Master
Powerdown signal forces the device to power up, and the
ISL4270E returns to enhanced automatic powerdown
6
FN6041.2
June 16, 2010