LTC2932 Linear Technology, LTC2932 Datasheet - Page 9

• Manufacturer Part Number: LTC2932
• Description: Confi gurable Six Supply Monitor
• Manufacturer: Linear Technology

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APPLICATIONS INFORMATION
Supply Monitoring
The LTC2932 is a low power, high accuracy confi gurable
six supply monitoring circuit with six real-time monitor
outputs, a common reset output, a reset disable function
and selectable supply thresholds. An external capacitor
sets the reset timeout period. 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 LTC2932 asserts the reset and compara-
tor 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 the 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 LTC2932
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.
CC
). On power-up, V
CC
< (V3 – 1.0V) and V
CC
powers the drive circuits
CC
< 2.4V, the V3
The LTC2932 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
4.675V. The threshold is guaranteed to lie in the band be-
tween 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 LTC2932’s ±1.5% specifi cation improves
the reliability of the system over supervisors with wider
threshold tolerances.
Monitor Confi guration and Tolerance Selection
Select the LTC2932 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.
Figure 1. 1.5% Threshold Accuracy Improves System Reliability
THRESHOLD
±1.5%
BAND
4.675V
PG
4.75V
4.6V
/V
5V
REF
Figure 2. Mode Selection
MINIMUM
RELIABLE
LTC2932
VOLTAGE
SYSTEM
REGION OF POTENTIAL MALFUNCTION
ratios (± 0.01), when confi guring with
VREF
GND
VPG
SUPPLY TOLERANCE
1%
R1
SUPERVISOR
THRESHOLD
2932 F02
IDEAL
R2
1%
LTC2932
NOMINAL
SUPPLY
VOLTAGE
–5%
–6.5%
–8%
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