Global Mixed-mode Technology Inc.
Output Ground Switches
Several PCMCIA power distribution switches on the
market do not have an active grounding FET switch.
These devices do not meet the PC Card specification
requiring a discharge of V
CC
within 100ms. PC Card
resistance can not be relied on to provide a discharge
path for voltages stored on PC Card capacitance be-
cause of possible high impedance isolation by power
management schemes. A method commonly shown to
alleviate this problem is to add to the switch output an
external 100kΩ resistor in parallel with the PC Card.
Considering that this is the only discharge path to
ground, a timing analysis show that the RC time con-
stant delays the required discharge time to more than
2 seconds. The only way to ensure timing compatibility
with PC Card standards is to use a power-distribution
switch that has an internal ground switch, like that of
the G570, or add an external ground FET to each of
the output lines with the control logic necessary to se-
lect it.
In summary, the G570 is a complete single-chip
dual-slot PC Card power interface. It meets all cur-
rently defined PCMCIA specifications for power deliv-
ery in 5V, 3.3V, and mixed systems, and offers a serial
control interface. The G570 offers functionality, power
savings, overcurrent and thermal protection, and fault
reporting in one 30 pin SSOP surface-mount package
for maximum value added to new portable designs.
Power Supply Considerations
The G570 has multiple pins for each of its 3.3V, 5V,
and 12V power inputs and for switched V
CC
outputs.
Any individual pin can conduct the rated input or out-
put current. Unless all pins are connected in parallel,
the series resistance is significantly higher than that
specified, resulting in increased voltage drops and lost
power. Both 12V inputs must be connected for proper
V
PP
switching; it is recommended that all input and
output power pins be paralleled for optimum operation.
Although the G570 is fairly immune to power input
fluctuations and noise, it is generally considered good
design practice to bypass power supplies typically with
a 1µF electrolytic or tantalum capacitor paralleled by a
0.047µF to 0.1µF ceramic capacitor. It is strongly re-
commended that the switched V
CC
and V
PP
outputs be
bypassed with a 0.1µF or larger capacitor; doing so
improves the immunity of the G570 to electrostatic
discharge (ESD). Care should be taken to minimize
the inductance of PCB traces between the G570 and
the load. High switching currents can produce large
negative-voltage transients, which forward biases
substrate diodes, resulting in unpredictable perform-
ance. Similarly, no pin should be taken below –0.3V.
G570
RESET or RESET Inputs
To ensure that cards are in a known state after power
brownouts or system initialization, the PC Cards
should be reset at the same time as the host by ap-
plying a low impedance to the V
CC
and V
PP
terminals.
A low impedance output state allows discharging of
residual voltage remaining on PC Card filter capaci-
tance, permitting the system (host and PC Cards) to
be powered up concurrently. The RESET or RESET
input closes internal switches S1, S4, S7, and S10
with all other switches left open (see G570 control
logic table). The G570 remains in the low impedance
output state until the signal is deasserted and further
data is clocked in and latched. RESET or RESET is
provided for direct compatibility with systems that use
either an active-low or active-high reset voltage super-
visor. The unused pin is internally pulled up or down
and should be left unconnected.
Overcurrent and Thermal Protection
The G570 uses sense FETs to check for overcurrent
conditions in each of the V
CC
and V
PP
outputs. Unlike
sense resistors or polyfuses, these FETs do not add to
the series resistance of the switch; therefore, voltage
and power losses are reduced. Overcurrent sensing is
applied to each output separately. When an overcur-
rent condition is detected, only the power output af-
fected is limited; all other power outputs continue to
function normally. The OC indicator, normally a logic
high, is a logic low when any overcurrent condition is
detected, providing for initiation of system diagnostics
and/or sending a warning message to the user.
During power up, the G570 controls the rise time of
the V
CC
and V
PP
outputs and limits the current into a
faulty card or connector. If a short circuit is applied
after power is established (e.g., hot insertion of a bad
card), current is initially limited only by the impedance
between the short and the power supply. In extreme
cases, as much as 10A to 15A may flow into the short
before the current limiting of the G570 engages. If the
V
CC
or V
PP
outputs are driven below ground, the G570
may latch nondestructively in an off state. Cycling
power will reestablish normal operation.
Overcurrent limiting for the V
CC
outputs is designed to
activate, if powered up, into a short in the range of
0.8A to 2.2A. The V
PP
outputs limit from 120mA to
400mA. The protection circuitry acts by linearly limiting
the current passing through the switch rather than ini-
tiating a full shutdown of the supply. Shutdown occurs
only during thermal limiting.
Thermal limiting prevents destruction of the IC from
overheating if the package power-dissipation ratings are
exceeded. Thermal limiting disables all power outputs
(both A and B slots) until the device has cooled.
Ver 1.0
Nov 09, 2000
TEL: 886-3-5788833
http://www.gmt.com.tw
9
GMT [ GLOBAL MIXED-MODE TECHNOLOGY INC ]
