LINEAR TECHNOLOGY LTC2932 Technical data

L DESIGN FEATURES

R1

1%

R2
1%

VREF

VPG

GND

LTC2931

+
–

0.5V

V3, V4,

V5 OR V6

V

TRIP

R3
1%

R4
1%

LTC2931

V4

VREF

V

TRIP

R4
1%

R3
1%

LTC2931

6-Input Supervisors Offer Accurate
Monitoring and 125°C Operation

by Shuley Nakamura and Al Hinckley

Introduction

The latest trio of power supply supervi­sors from Linear Technology is ideal
for today’s multi-voltage systems that
require accurate supply monitoring.
The LTC2930, LTC2931, and LTC2932
are 6-input voltage monitors capable of
maintaining 1.5% threshold accuracy
from –40°C to 125°C. The combina­tion of monitored supply voltages is
set by a single pin. Each part offers
16 threshold voltage combinations,
thus meeting the needs of almost
any multi-voltage system. This pro­grammability eliminates the need to
qualify, source and stock unique part
numbers for different threshold voltage
combinations.

The overall architecture and op­erating specifications of these three
devices are similar, but each has
unique features (see Table 1). The
LTC2930 generates a reset after
any undervoltage event or when the
manual reset input (MR) pulls low. It
is ideal for space-constrained applica­tions as it comes in a compact 3mm
× 3mm 12-lead DFN package. The
LTC2931 includes a watchdog input
(WDI), a watchdog output (WDO) and
user-adjustable watchdog periods to
enable microprocessor monitoring
and control. The LTC2932 can vary
its monitor thresholds from 5% to

12.5%, and a reset disable pin pro­vides margining capability. Both the
LTC2931 and LTC2932 are packaged
in 20-pin TSSOP packages and have
separate comparator outputs, en­abling individual supply monitoring
and/or sequencing.

Feature LTC2930 LTC2931 LTC2932

Configurable Input

Threshold Combinations

Threshold Accuracy 1.5% 1.5% 1.5%

Adjustable Reset Time

Buffered Reference

Individual Comparator

Outputs

Manual Reset

Independent Watchdog

Circuitry

Reset Disable

Supply Tolerance Fixed, 5% Fixed, 5%

Package

Single Pin Configuration
Makes Life Easy

These supervisors offer an elegant
method of configuring the input volt­age thresholds. Figure 1 shows how a
single resistive divider at the VPG pin
sets the supervisor into one of the 16
threshold options shown in Table 2.
See the data sheet for suggested mode­setting resistor values.

The actual thresholds are set by

integrated precision dividers for 5V,

Table 1. LTC2930, LTC2931, LTC2932 feature summary

16 16 16

L L L

L L L

L L

L

L

User Selectable

5%, 7.5%, 10%, 12.5%

12-lead

3mm × 3mm DFN

20-lead

F Package

3.3V, 3V, 2.5V, 1.8V, and 1.5V supply
monitoring. For other supply values,
uncommitted comparators with 0.5V
thresholds allow virtually any positive
supply to be monitored using a resis­tive divider, as shown in Figure 2a.
The V4 input also monitors negative
voltages—with the same 1.5% accu­racy—using the integrated buffered
reference for offset (see Figure 2b).

20-lead

F Package

L

10

Figure 1. Mode selection

2a. 2b.

Figure 2. Using a resistive divider to set the voltage trip point

Linear Technology Magazine • March 2008

DESIGN FEATURES L

5V

4.75V

4.675V

±1.5%

THRESHOLD

BAND

4.6V

NOMINAL
SUPPLY
VOLTAGE

SUPPLY TOLERANCE

MINIMUM
RELIABLE

SYSTEM

VOLTAGE

IDEAL

SUPERVISOR

THRESHOLD

REGION OF POTENTIAL MALFUNCTION

–5%

–6.5%

–8%

What Does Threshold
Accuracy Mean?

Consider a 5V system with ±5% sup­ply 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 fluctuates over a specified band;
the LTC2930, LTC2931 and LTC2932
vary ±1.5% around their nominal
threshold voltage over temperature
(Figure 3). 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 specified
tolerance band.

The L TC 2930, LT C293 1 and
LTC2932 boast a ±1.5% reset thresh-
old accuracy, so a “5%” threshold is
usually 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 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 tight ±1.5% accuracy
specification of the LTC2930, LTC2931

Linear Technology Magazine • March 2008

Figure 3. Tight 1.5% threshold accuracy yields high system reliability

Table 2. Voltage threshold modes

V1 (V) V2 (V) V3 (V) V4 (V) V5 (V) V6 (V)

5.0 3.3 2.5 1.8 ADJ ADJ

5.0 3.3 2.5 1.5 ADJ ADJ

5.0 3.3 2.5 ADJ ADJ ADJ

5.0 3.3 1.8 ADJ ADJ ADJ

5.0 3.3 1.8 –ADJ ADJ ADJ

5.0 3.3 ADJ ADJ ADJ ADJ

5.0 3.3 ADJ –ADJ ADJ ADJ

5.0 3.0 2.5 ADJ ADJ ADJ

5.0 3.0 1.8 ADJ ADJ ADJ

5.0 3.0 ADJ ADJ ADJ ADJ

3.3 2.5 1.8 1.5 ADJ ADJ

3.3 2.5 1.8 ADJ ADJ ADJ

3.3 2.5 1.8 –ADJ ADJ ADJ

3.3 2.5 1.5 ADJ ADJ ADJ

3.3 2.5 ADJ ADJ ADJ ADJ

3.3 2.5 ADJ –ADJ ADJ ADJ

and LTC2932 improves the reliability
of the system over supervisors with
wider threshold specifications.

coupling from other signals. If the
monitored voltage is near or at the re­set threshold voltage, this noise could
cause spurious resets. Fortunately,

Glitch Immunity =
No Spurious Resets!

Monitored supply voltages are far from
being ideal, perfectly flat DC signals.
Riding on top of these supplies are
high frequency components caused
by a number of sources such as the
output ripple of the power supply or

the LTC2930, LTC2931 and LTC2932
have been designed with this potential
issue in mind, so spurious resets are
of little to no concern.

Some supply monitors overcome
spurious resets by adding hysteresis
to the input comparator. The amount
of applied hysteresis is stated as
a percentage of the trip threshold.
Unfortunately, this degrades monitor
accuracy because the true accuracy
of the trip threshold is now the per­centage of added hysteresis plus the
advertised accuracy of the part. The
LTC2930, LTC2931 and LTC2932 do
not use hysteresis, but instead use
an integration scheme that requires
transients to possess enough mag­nitude and duration to switch the
comparators. This suppresses spu­rious resets without degrading the
monitor accuracy.

The COMP5 comparator output
response to a “noisy” input on the
LTC2931 is demonstrated in Figure 4.

11

L DESIGN FEATURES

C

t

M

pF ms t

RT

RST

RST

= =

( )

2

500

Ω

•

t

RST

V

RT

V

n

RST

COMP

n

t

UV

300µs PROPAGATION DELAY

–2mV DC STEP APPLIED HERE

500mV

100mV

P–P

V5

100mV/DIV

COMP5

LTC2930

1V

0.9V

–5.2V

1.8V

3.3V

5V

C

RT

47nF

MR

RST

VREF

VPG

GND CRT

V1

V2

V3

V4

V5

V6
121k
1%

R1A

16.2k
1%

R2A

86.6k
1%

100k
1%

10k**

10k

100k
1%

487k
1%

86.6k
1%

68.1k
1%

0.1MF
LTC2930

6V

8V

12V

3V

5V

C

RT

47nF

MANUAL RESET
PUSHBUTTON

t

RST

= 94ms

**OPTIONAL FOR EXTENDED ESD TOLERANCE

MR

RST RESET

VREF

VPG

GND CRT

V1

V2

V3

V4

V5

V6

100k
1%

R1B

40.2k
1%

R2B

59k

1%

100k
1%

100k
1%

2150k
1%

1400k
1%

1020k
1%

0.1MF

Figure 4. Comparator output is
resistant to noisy input voltage

In the example shown, a 500kHz,
100mV

sine wave centered at

P–P

500mV is applied to the V5 input. The
threshold voltage of the adjustable
input, V5, is 500mV. Even though
the signal amplitude goes as low as
450mV, COMP5 remains high. Next,
the DC level of the input is dropped
2mV. In response, COMP5 pulls low
and remains low. As mentioned earlier,
only transients of long enough duration
and magnitude trigger the comparator
output to pull high or low.

Adjustable Reset Timeout Period
for Varied Application Needs

Each of the supervisors includes an
adjustable reset timeout period, t

RST

.
Once all the inputs are above their
threshold values, the reset timer is
started (Figure 5). RST stays low for

Figure 5. RST timing diagram

the duration of t

and remains low

RST

as long as the time between transients
is less than the reset timeout. In other
words, the reset timeout prevents sup­ply transients with frequencies greater
than 1/t

from causing undesired

RST

toggling at the RST output. Keeping
RST low during these supply transients
suppresses spurious resets.

The reset timeout period is adjust­able to accommodate a variety of
microprocessor applications. Config­ure the reset timeout period, t

RST

, by
connecting a capacitor, CRT, between
the CRT pin and GND. The value of
this capacitor is determined by

Leaving the CRT pin unconnected

generates a minimum reset timeout of

approximately 25µs. Maximum reset
timeout is limited by the largest avail­able low leakage capacitor.

Additional Glitch Filtering

Even though all six comparators
have built-in glitch filtering, adding
bypass capacitors on the V1 and V2
inputs is recommended, because of
these two, the input with the higher
voltage functions as VCC for the entire
chip. Additional filter capacitors may
be added to the V3, V4, V5 and V6
inputs if needed to suppress trouble­some noise.

Open-Drain Reset

The RST outputs on the LTC2930,
LTC2931 and LTC2932 are open­drain and contain weak pull-up
current sources to the V2 voltage.

Figure 6. Wired-OR system reset

12

Linear Technology Magazine • March 2008