LTC2955
17
2955fa
For more information www.linear.com/LTC2955
applicaTions inForMaTion
If a higher V
EN(VOH)
is required, an external pull-up resistor
can be connected from the EN pin to any higher voltage
(<36V). The EN pin is designed to be able to sink at least
1mA of current during turn-off, so this external pull-up
resistor value must be selected to source less than 1mA
with EN at 0V.
The LTC2955-1 EN pin can also be connected to switching
regulators with a RUN/SS pin. RUN/SS has a dual func
-
tion of a SHUTDOWN t
hre
shold and soft-start, with an
internal soft-start pull-up current and an external soft-start
capacitor. The LTC2955 EN pin pull-up current of 2µA will
add to the soft-start pull-up current of the regulator. The
and the device turns on. If the user slides the switch to
the OFF position, the ON pin voltage drops to 0V and the
LTC2955 activates a turn-off since the SEL pin is high
(DFN package only).
Interface with Switching Regulators
The LTC2955-1 EN pin can be connected directly to most
switching regulator SHUTDOWN inputs. The EN pin high
level output voltage (V
EN(VOH)
) is typically 4.3V with
V
IN
>5V, and V
EN(VOH)
= V
IN
– 0.5V if V
IN
<5V. With a
minimum V
IN
of 1.5V, V
EN(VOH)
is still higher than most
SHUTDOWN thresholds (usually <1.2V). Figure 16 shows
one such application. The regulator is a boost converter
with a SHUTDOWN high threshold of 0.88V (maximum).
Figure 16. 2-Cell with 3.3V Output
Figure 17. Turn-On/Off a DC/DC Regulator Through the
RUN/SS Pin
Figure 18. Recommended Layout for the V
IN
Bypass
Capacitor
LTC3528
V
OUT
FB
V
IN
PGOOD
SHDN
V
IN
PB
INT
KILL
V
IN
TMR
LTC2955TS8-1
2955 F16
499k
GND
ON
EN
V
OUT
3.3V
0.4A
287k
GND
SW
4.7µF
1.5V TO 3.2V
1M
22pF
22µF
4.7µH
DC/DC
V
OUT
RUN/SS
V
OUT
GND
C
SS
C
OUT
V
IN
V
IN
PB
INT
V
IN
LTC2955TS8-1
2955 F17
µP
1M
GND
ON
EN
TMR
KILL
C
IN
soft-start capacitor may need to be increased to maintain
the same soft-start time. The soft-start behavior of the
regulator will remain the same.
Figure 17 shows the LTC2955-1 EN pin connected to a
DC/DC regulator RUN/SS pin.
Layout Considerations
Figure 18 shows example PCB layouts for placing the V
IN
pin bypass capacitor. Position the bypass capacitor close
to the LTC2955 on the same side of the PCB and keep the
traces short. For the DFN package, a PCB via should be
placed near the LTC2955 to connect the PB pin (Pin 5)
to the pushbutton. This allows an unbroken trace to be
p
lace
d between the V
IN
pin and the bypass capacitor.
Placing the bypass capacitor close to the LTC2955 gives
the optimum protection against PB pin ESD transients.
2955 F18
GND
C
VIN
11
6
8
7
9
10
5
4
2
3
1
V
V
IN
1
2
3
4
8
7
6
5
GND
C
VIN
