5
FN6413.1
April 22, 2009
ISL8723, ISL8724 Descriptions and
Operation
The ISL8723 and ISL8724 sequencers are quad voltage
sequencing controllers designed for use in multiple-voltage
systems requiring power sequencing of various supply
voltages. Individual voltage rails are gated on and off by
external N-Channel MOSFETs, the gates of which are
driven by an internal charge pump to ~V
DD
+5.6V (VQP) in
a user programmed sequence.
With the ISL8723, the ENABLE must be asserted high and
all four voltages to be sequenced must be above their
respective user programmed Undervoltage Lock Out
(UVLO) levels before programmed output turn on
sequencing can begin. Sequencing and delay
determination is accomplished by the choice of external
capacitor values on the DLY_ON and DLY_OFF pins. The
SYSRST
goes high once all 4 UVLO inputs and ENABLE
are satisfied. Once all 4 UVLO inputs and ENABLE are
satisfied for 10ms, the four DLY_ON capacitors are
simultaneously charged with 1µA current sources to the
DLY_Vth level of 1.28V. As each DLY_ON pin reaches the
DLY_Vth level, its associated GATE will then turn-on with a
10µA source current to the VQP voltage of V
DD
+5.6V.
Thus, all four GATEs will sequentially turn on. Once at
DLY_Vth the DLY_ON pins will discharge to be ready when
next needed. After the entire turn on sequence has been
completed and all GATEs have reached the charge
pumped voltage (VQP), a 160ms delay is started to ensure
stability after which the RESET
output will be released to go
high. Subsequent to turn-on, if any input falls below its
UVLO point for longer than the glitch filter period, t
FIL
(~7µs) this is considered a fault. RESET
, SYSRST and all
GATEs are simultaneously pulled low. In this mode the
GATEs are pulled low with ~75mA. Normal shutdown mode
is entered when no UVLO is violated and the ENABLE is
deasserted. When ENABLE is deasserted, RESET
is
asserted and pulled low. Next, all four shutdown ramp
capacitors on the DLY_OFF pins are charged with a 1µA
source and when any ramp-capacitor reaches DLY_Vth, a
latch is set and a 10µA current is sunk on the respective
GATE pin to turn off its external MOSFET. When the falling
GATE voltage is approximately 1.5V, the GATE is pulled
down the rest of the way at a higher current level to ensure
a hard turn-off. Each individual external FET is thus turned
off removing the voltages from the load in the programmed
sequence. The SYSRST
will pull low concurrent with the
last GATE being pulled low.
The ISL8723 and ISL8724 have the same functionality
except for the complimentary ENABLE active polarity with
the ISL8724 having an ENABLE
input. Additionally, the
ISL8723 also has a low power sleep state when disabled.
Upon bias, the SYSRST
and RESET pins are held low
before bias voltage = 1V.
The SYSRST
has both an input and output function. As an
output, the SYSRST
pin is useful when implementing
multiple sequencers in a design needing simultaneous
shutdown as with a kill switch across all sequencers. Once
any UVLO is unsatisfied for longer than t
FIL
, the related
SYSRST
will pull low and pull all other SYSRST pins low
SYSRST Low Output Voltage V
OL_SRST
V
DD
= 5V, I
OUT
= 100μA- 0.1V
SYSRST
Output Capacitance C
OUT_SRST
-10-pF
SYSRST
Low to GATE Turn-off t
delSYS_G_1
GATE = 80% of V
DD
+5V - 40 - ns
SYSRST High to GATE Turn-on t
delSYS_G_2
GATE = 50% of V
DD
+5V - 0.4 - ms
GATE
GATE Turn-On Current I
GATEon
GATE = 0V 8.3 10.2 12.5 µA
GATE Turn-Off Current I
GATEoff_l
GATE = V
DD
, Disabled -12.5 -10.2 -8.3 µA
GATE Current Range I
GATE_range
Within IC I
GATE
max-min - 0.6 3 µA
GATE Pull-Down High Current I
GATEoff_h
GATE = V
DD
, UVLO = 0V - 75 - mA
GATE High Voltage V
GATEh5
V
DD
= 5V V
DD
+5.3V V
DD
+5.6V - V
GATE Low Voltage V
GATEl
Gate Low Voltage, V
DD
= 1V - 0.01 0.1 V
BIAS
IC Supply Current I
VDD_5V
V
DD
= 5V, Enabled and static - 0.48 0.6 mA
ISL8723 Stand By IC Supply Current I
VDD_sb
V
DD
= 5V, ENABLE = 0V - 30 40 µA
V
DD
Power On Reset V
DD
_POR V
DD
rising - 2.2 2.41 V
Electrical Specifications V
DD
= 3.3V to +5V, T
A
= T
J
= -40°C to +85°C, Unless Otherwise Specified. Parameters with MIN and/or MAX
limits are 100% tested at +25°C, unless otherwise specified. Temperature limits established by characterization
and are not production tested. (Continued)
PARAMETER SYMBOL TEST CONDITIONS MIN TYP MAX UNIT
ISL8723, ISL8724
