LTC2908
APPLICATIO S I FOR ATIO
Supply Monitoring
The LTC2908 is a low power, high accuracy, six input
supply monitoring circuit with two adjustable inputs. The
reset delay is set to a nominal of 200ms with an internal
capacitor, eliminating the need for an external timing
capacitor.
All input voltages must be above predetermined thresh-
olds for the reset not to be invoked. The LTC2908 asserts
the reset output during power-up, power-down and brown-
out conditions on any one of the voltage inputs.
Ultralow Voltage Pull-Down on RST
The LTC2908 issues a logic low on the RST output when
any one of the inputs falls below its threshold. Ideally, the
RST logic output would remain low with the input supply
voltage down to zero volts. Most supervisors lack pull-
down capability below 1V.
The LTC2908 power supply supervisor incorporates a
novel low voltage pull-down circuit that can hold the RST
line low with as little as 200mV of input supply voltage on
V1 and/or V2 (see Figures 1 and 2). The pull-down circuit
helps maintain a low impedance path to ground, reducing
the risk of the RST node from floating to an indeterminate
voltage.
10
V
CC
= V1 = V2
V3 = V4 = V
ADJ1
= V
ADJ2
= GND
RST AT 150mV
RST PULL-DOWN CURRENT, I
RST
(mA)
1
RST PULL-DOWN CURRENT, I
RST
(mA)
0.1
RST AT 50mV
0.01
0.001
0
0.4
0.6
0.8
0.2
SUPPLY VOLTAGE, V
CC
(V)
1
2908 G16
Figure 1. RST Pull-Down Current (I
RST
) vs
Supply Voltage (V
CC
) with Dual Channel Supply
8
U
Such an indeterminate voltage may trigger external logic
causing erroneous reset operation(s). Furthermore, a
mid-scale voltage level could cause external circuits to
operate in the middle of their voltage transfer character-
istic, consuming more quiescent current than normal.
These conditions could cause serious system reliability
problems.
Power-Up
During power-up, RST starts asserting low as soon as
there is at least 200mV on V1 and/or V2. The RST pull-
down capability is a function of V1 and V2 as shown in the
Typical Performance Characteristics.
The greater of V1, V2 is the internal supply voltage (V
CC
)
that powers the other internal circuitry. Once all the V
X
inputs rise above their thresholds, an internal timer is
started. After the internal timer counts a 200ms delay time,
RST weakly pulls high to V
CC
.
Power-Down
On power-down, once any of the V
X
inputs drop below
their threshold, RST asserts logic low. V
CC
of at least 0.5V
guarantees a logic low of 0.15V at RST.
10
V
CC
= V1
V2 = V3 = V4 = V
ADJ1
= V
ADJ2
= GND
1
RST AT 150mV
0.1
RST AT 50mV
0.01
0.001
0
0.4
0.6
0.8
0.2
SUPPLY VOLTAGE, V
CC
(V)
1
2908 G17
W
U U
Figure 2. RST Pull-Down Current (I
RST
) vs
Supply Voltage (V
CC
) with Single Channel Supply
sn2908 2908fs
LINEAR [ LINEAR INTEGRATED SYSTEMS ]
