QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1090A
PUSH BUTTON ON/OFF CONTROLLER WITH MICROPROCESSOR INTERRUPT
2
OPERATING PRINCIPLES
On the DC1090A, a push button switch shorts the /PB
pin to ground which in turn sets the /EN pin low.
Shorting /PB to ground a second time and holding
subsequently resets the /EN pin high. The /EN pin is
used to drive the gate of a P-channel MOSFET to con-
trol the power path to a DC/DC converter. The turn on
and off of the circuit is displayed with a green LED
(D2).
The duration that /PB must be shorted to ground in
order to turn on/off the DC/DC converter is independ-
ently programmed by two external capacitors C1 and
C2 and selected on the DC1090A with jumper JP1 and
JP2.
An internal 500ms timer blanks (ignores) the /KILL
signal during system power up. This allows sufficient
time for the DC/DC converter and a µP to perform
power up tasks. During turn off, a power down timer
provides a delay from interrupting the µP (/INT=low)
to turning off the DC/DC converter (/EN=high). This
delay gives the µP time to perform power down and
housekeeping tasks. On the DC1090A, JP1 selects the
turn on timer, while JP2 is used to adjust the power
down /PB duration. The red LED D1 shows the state
of the /INT pin. A µP can turn off the converter with
no delay by asserting /KILL low.
By tying /KILL to /INT through JP3, /KILL is forced low
during the /INT blanking time and thus forces a turn
off.
The RC (R
RPP
and C
RPP
) at VIN on the DC1090A provides
a reverse polarity protection to the LTC2954-2. If
power is connected in an application such that the po-
larities are ensured to be in the correct configuration,
this RC may be removed from the circuit.
An additional RC (R
F
and C
F
), located at the /PB pin,
acts as a filter and used in an application where the
switch is located far from the LTC2954-2. In such a
case, the RC would be placed next to the switch rather
than the part. If the switch is located near the
LTC2954-2, then the RC may be removed from the
circuit.
An LDO (U2) is used on the DC1090 to provide a pull-
up voltage of 2.6V which is well below the absolute
maximum on the /INT, /EN, and /KILL pins for the full
range of input voltage of 2.7V to 26.4V. In an applica-
tion, pull these pins up to a voltage no higher than
their rated absolute maximum shown in the data
sheet. (If the interface pins on the DC1090A are pulled
up to an external supply higher than 5V, LEDs D1 and
D2 should be removed.)
Table 1.
DC1090A Typical Performance Summary (T
A
= 25°C)
PARAMETER
CONDITION
VALUE
Input Voltage
V
IN
2.7V to 26.4V
Output Voltage
V
OUT
, /PB Turn On
V
2P6V
, /PB Turn On
V
IN
- V
DS
2.6V ± 2%
/PB Turn On Time
JP1 on OPEN
JP1 on CAP*
32ms
32 + 212ms
/PB Turn Power Down Time
JP2 on OPEN
JP2 on CAP*
32ms
32 + 3014ms
*The additional Cap. Adjust time is selected by an external capacitor. The DC1090A adjust times have been pre-selected with 0.033uF at ONT and 0.47uF at CPD to provide
additional 212ms and 3014ms turn on and turn power down time respectively. The additional time is calculated with the following equations as shown in the LTC2954 data
sheet:
C
ONT
= 1.56 x 10
-4
[
µ
F/ms] • (t
ONT
– 1ms)
C
PDT
= 1.56 x 10
-4
[
µ
F/ms] • (t
PDT
– 1ms)
