LTC2931CF#PBF Linear Technology, LTC2931CF#PBF Datasheet - Page 9

• Manufacturer Part Number: LTC2931CF#PBF
• Description: IC 6 PWR SUPPLY MONITOR 20TSSOP
• Manufacturer: Linear Technology

Specifications of LTC2931CF#PBF

• Applications: Six Power Supply Monitor
• Voltage - Supply: 1 V ~ 7 V
• Operating Temperature: 0°C ~ 70°C
• Mounting Type: Surface Mount
• Package / Case: 20-TSSOP
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Current - Supply: -
• Voltage - Input: -

APPLICATIONS INFORMATION
Supply Monitoring
The LTC2931 is a low power, high accuracy confi gurable
six supply monitoring circuit with six real-time monitor
outputs, a common reset output and a watchdog timer.
External capacitors set the reset and watchdog timeout
periods. An external resistive divider between VREF , VPG
and GND selects 1 of 16 possible input voltage monitor
combinations. All six voltage inputs must be above their
predetermined thresholds for the reset not to be activated.
The LTC2931 asserts the reset and comparator outputs
during power-up, power-down and brownout conditions
on any one of the voltage inputs.
Power-Up
The greater of V1 and V2 serves as the internal supply
voltage (V
for the RST pin. This ensures that the RST output will be
low as soon as either V1 or V2 reaches 1V. The RST output
remains low until the part is confi gured. Once voltage
thresholds are set, if any of the supply monitor inputs is
below its confi gured threshold, RST will be a logic low.
Once all the monitor inputs rise above their thresholds,
an internal timer is started and RST is released after the
delay time. If V
input impedance will be low (10kΩ typical).
Threshold Accuracy
Consider a 5V system with ±5% tolerance. The 5V supply
may vary between 4.75V to 5.25V. System ICs powered
by this supply must operate reliably within this band (and
a little more as explained below). A perfectly accurate
supervisor for this supply generates a reset at exactly
4.75V, however no supervisor is this perfect. The actual
reset threshold of a supervisor varies over a specifi ed band;
the LTC2931 varies ±1.5% around its nominal threshold
voltage (see Figure 1) over temperature.
The reset threshold band and the power supply tolerance
bands should not overlap. This prevents false or nuisance
resets when the power supply is actually within its speci-
fi ed tolerance band.
The LTC2931 has a ±1.5% reset threshold accuracy, so a
“5%” threshold is typically set to 6.5% below the nominal
input voltage. Therefore, a typical 5V, “5%” threshold is
CC
). On power-up, V
CC
< (V3 – 1.0V) and V
CC
powers the drive circuits
CC
< 2.4V, the V3
THRESHOLD
4.675V. The threshold is guaranteed to lie in the band
between 4.750V and 4.600V over temperature. The
powered system must work reliably down to the low end
of the threshold band, or risk malfunction before a reset
signal is properly issued.
A less accurate supervisor increases the required system
voltage margin and increases the probability of system
malfunction. The LTC2931 ±1.5% specifi cation improves
the reliability of the system over supervisors with wider
threshold tolerances.
Monitor Confi guration
Select the LTC2931 input voltage combination by placing
the recommended resistive divider from VREF to GND
and connecting the tap point to VPG, as shown in Figure
2. Table 1 offers recommended 1% resistor values for
each of the 16 modes. The last column in Table 1 specifi es
optimum V
a ratiometric DAC.
At power-up, once V1 or V2 reaches 2.4V, the monitor
enters a setup period of approximately 150μs. During the
setup time, the voltage on the VPG pin is sampled and
the monitor is confi gured to the desired input combina-
Figure 1. 1.5% Threshold Accuracy Improves System Reliability
±1.5%
BAND
4.675V
4.75V
PG
4.6V
5V
/V
REF
LTC2931
Figure 2. Mode Selection
MINIMUM
RELIABLE
VOLTAGE
SYSTEM
REGION OF POTENTIAL MALFUNCTION
ratios (± 0.01), when confi guring with
VREF
GND
VPG
SUPPLY TOLERANCE
1%
R1
SUPERVISOR
THRESHOLD
R2
1%
2931 F02
IDEAL
LTC2931
NOMINAL
SUPPLY
VOLTAGE
–5%
–6.5%
–8%
2931fb
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