Datasheet

73S8009C Data Sheet DS_8009C_025
18 Rev. 1.5
3.2 Power Supply and Converter
The power supply and converter circuit takes power from any one of three sources; V
PC
, V
BUS
, and V
BAT
.
V
PC
is specified to range from 2.7 to 6.5 volts and would typically be supplied by a single cell battery with
a voltage range of 2.7 to approximately 3.1 volts. V
PC
is also appropriate for system supplies of 3.3 or 5
volts. V
BUS
is typically supplied by a connected USB cable and ranges in value from 4.5 to 5.5 volts
(6.5 V maximum). V
BAT
is expected to be supplied from a battery of two series connected cells with a
voltage value of 4.0 V to 6.5 V. V
BAT
and V
BUS
are connected to V
PC
by two FET switches configured as
an SPDT switch (break-before-make). They are not enabled at the same time. V
BUS
is automatically
selected in lieu of V
BAT
when V
BUS
is present. If V
PC
is provided and V
BAT
or V
BUS
are also used, the
source of V
PC
must be diode isolated from the V
PC
pin to prevent current flow from V
BAT
or V
BUS
into the
V
PC
source.
The power supplied to the V
PC
is up-converted to the voltage V
P
utilizing an inductive, step-up converter.
A series power inductor (nominal value = 10 µH) is connected from V
PC
to LIN, and a 10 µF filter capacitor
plus a 0.1 µF capacitor must be connected to V
PC
. V
P
requires a 4.7 µF filter capacitor and will have a
nominal value of 5.5 V during normal operation. V
P
is used by the smartcard interface circuits (CLK, RST,
IO, AUX1, and AUX2) and is the source of the regulated smart card supply V
CC
. V
CC
can be programmed
for values of 5 V, 3 V, and 1.8 V. V
DD
is also produced from V
P
. V
DD
is used by the 73S8009C circuit for
logic, input/output buffering, and analog functions as well as being capable of supplying up to 40 mA of
current to external devices. Figure 4 shows the block diagram of the 73S8009C.
3.3 Power ON/OFF
When no power is applied to the V
BUS
pin, a power ON/OFF function is provided such that the circuit will
be inoperative during the “OFF” state, consuming minimum current from V
PC
and V
BAT
. If V
BUS
power is
supplied, the functions of the ON/OFF switch and circuitry are overridden and the 73S8009C is in the
“ON” state with V
P
and V
DD
available. Without V
BUS
applied, and in the OFF state, the circuit responds
only to the ON_OFF pin. The ON_OFF pin shall be connected to a SPST switch to ground. If the circuit
is OFF and the switch is closed for a de-bounce period of 50-100ms, the circuit shall go into the “ON”
state wherein all functions are operating in normal fashion. If the circuit is in the “ON” state and the
ON_OFF pin is connected to ground for a period greater than the de-bounce period, OFF_REQ will be
asserted high and held. Typically, the OFF_REQ signal is presented to a host controller that will assert
OFF_ACK high when it has completed all shutdown activities. When OFF_ACK is set high, the circuit will
de-activate the smart card interface if required and turn off all analog functions and the V
DD
supply for the
logic and companion circuits. The OFF_ACK pin is connected internally to OFF_REQ with a resistor such
that if OFF_ACK is unconnected, the action of OFF_REQ will assert OFF_ACK high. In this
configuration, the circuit shall go into the “OFF” state immediately upon OFF_REQ = 1. The default state
upon application of power is the “OFF” state unless power is supplied to the V
BUS
supply. Note that at any
time, the controller may assert OFF_ACK and the 73S8009C will go into the “OFF” state (when V
BUS
is
not present.)
If power is applied to both V
BAT
and V
BUS
, the circuit will automatically consume power from only the V
BUS
source. The circuit will be unconditionally “ON” when V
BUS
is applied. If the V
BUS
source is removed, the
circuit will switchover to the VBAT input supply and remain in the “ON” state. The controller circuit
firmware is required to assert OFF_ACK based on no activity or V
BUS
removal to reduce battery power
consumption. When operating from V
BUS
, and not calling for V
CC
, the step-up converter becomes a simple
switch connecting V
BUS
to V
P
in order to save power. This condition is appropriate for the USB
“SUSPEND” state. The USB “SUSPEND” state requires the power supply current to be less than 500 µA.
In order to obtain and meet this low current limitation, the companion controller must be configured into a
power-down condition using less than 20 µA from V
DD
.
Note: When using the VBUS input as the sole power source for an ‘always on’ configuration
(ON_OFF input not used), the OFF_ACK and ON_OFF inputs must be connected to ground.