6
SMT4004
2049 2.2 9/13/00 SUMMIT MICROELECTRONICS, Inc.
The on-board electronic circuit breaker can be pro-
grammed to application specific levels. The circuit
breaker delay defines the period of time the voltage drop
across RS is greater than 25mV but less than VQCB before
the VGATE output will be shut down. This is effectively a
filter to prevent spurious shutdowns of VGATE. The
delays that can be programmed are 25µs, 50µs, 100µs
and 200µs. The programmable delay bits are located in
Register
R1B
.
The Quick-Trip circuit breaker threshold (VQCB) can be set
to 150mV, 100mV, 75mV or off (Register
R1A
). This is the
threshold voltage drop across RS that is placed between
VSS and CBSense. If the voltage drop exceeds the
programmed threshold, the electronic circuit breaker will
immediately trigger with no delay.
The outputs of these comparators can be used to generate
interrupts and reset conditions and toggle the crowbar
output.
POWER-ON SEQUENCING
In order to begin sequencing of the card-side supplies
(ramping the VGATE outputs) a number of conditions
must be met. All enabled bus-side voltages must be above
their respective under-voltage thresholds, the card-side
voltages (
e.g.
, residual capacitor stored potentials) must
be near zero volts, and the following inputs must be
properly set.
ENABLE — When active the ENABLE input brings
the IC out of a standby mode where the charge pump
supplying the VGATE outputs is turned on (and
begins charging the VGG_CAP) and the bandgap
reference is turned on. The ENABLE input can be
programmed to be either active low (default from the
factory) or active high (Register
R1B
).
SEATED1# and SEATED2# — the SEATED inputs
are effectively two additional enable inputs that must
be low to enable the sequencing of the card-side
voltages. In a staggered pin environment these
inputs can be tied to the “short” pins, insuring the card
is fully seated before any power is applied to the card-
side logic. These inputs can also be tied to card
insertion switches to indicate proper seating.
PWR_ON — the PWR_ON input is the last input that
will typically be driven to enable power sequencing to
the card-side. The PWR_ON input can be pro-
grammed to be either active low (default from the
factory) or active high (Register
R1B
).
TRAKKING
AND SOFTSTART CONTROL
VGATE — The VGATE outputs are used to control the
“turning-on” of the card-side voltages. The ramp rate (for
both turn-on and turn-off) of the outputs is programmable
from 100V/s to 1000V/s (Register
R10
). The four outputs
ramp at the same slew-rate, so normally there will be no
differential voltage between any of the supplies until each
reaches its maximum level.
The ramp rates are inherently adaptive. That is, if the
difference between any VO input is greater than 100mV in
the linear region, the slew rate will be increased or de-
creased to minimize the differential. The comparisons are
made between VO1 and VO2, VO2 and VO3, VO3 and
VO4, and VO4 and VO1. If at any time a differential of
greater than 300mV is detected a pre-programmed (Reg-
ister
R10
) action can be taken. The
TRAKKER
can shut
down the offending supply, generate an interrupt output, or
ignore the situation.
If SoftStart is enabled (Registers
R0C
through
R0F
) the
supply or supplies designated will be ramped as soon as
the input conditions are met and no Trakking will be
performed. Any supply not designated as a softstart
supply will not be ramped until the designated supply has
reached its VO threshold. This type of operation would
commonly be used where a bus voltage (
e.g.
, 5V) is first
switched to a DC-to-DC converter or group of LDOs; and
then their outputs would be switched in a Trakking mode
to the card-side logic.
Supply managers designated for Trakking will not begin
start-up until the soft start channels are fully turned on.
The delay is approximated by the formula tD =16,000 ÷ SR,
where tD is the time delay in milliseconds between the
PWR_ON signal going high and the start of the tracking
ramp-up, and SR is the programmed start-up slew rate in
V/s. For example, the time delay for a programmed slew
rate of 500V/s is: tD = 16,000 ÷ 500 = 32ms.
POWER MANAGEMENT STATUS OUTPUTS
The
TRAKKER
has two types of status outputs that it
provides to the host system or host processor resident on
its board. One type of output is “hardwired” internally and
the other is programmable.
HEALTHY# — The HEALTHY output is an active-low
open-drain output that can be wire-ORed with other open-
drain signals. It is driven low when all of the enabled
managers’ card-side voltages are valid and there are no
over-current conditions. The signal is used to indicate the
power supplies are within their programmed operating
limits.