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LTC2908 Precision Six Supply Monitor
FEATURES

DESCRIPTIO

Ultralow Voltage Reset: VCC = 0.5V Guaranteed* Monitors Six Inputs Simultaneously: 5V, 3.3V, 2.5V, 1.8V, ADJ1, ADJ2 (LTC2908-A1) 3.3V, 2.5V, 1.8V, 1.5V, ADJ1, ADJ2 (LTC2908-B1) Guaranteed Threshold Accuracy: 1.5% of Monitored Voltage Over Temperature Internal VCC Auto Select Power Supply Glitch Immunity 200ms Reset Time Delay Active Low Open-Drain RST Output Low Profile (1mm) SOT-23 (ThinSOTTM) and Plastic (2mm x 3mm) DFN Packages
The LTC(R)2908 is a six supply monitor for systems with a large number of supply voltages that require a precise and compact solution. The common reset output remains low until all six inputs have been in compliance for 200ms. The LTC2908 features a tight 1.5% threshold accuracy over the entire operating temperature range (- 40C to 85C) and glitch immunity to ensure reliable reset operation without false triggering. The open-drain RST output state is guaranteed to be in the correct state as long as V1 and/or V2 is 0.5V or greater. The LTC2908 also features two adjustable inputs with a nominal threshold level at 0.5V. This product provides a precise, space-conscious, micropower and general purpose solution for any kind of system requiring supply monitors.
, LTC and LT are registered trademarks of Linear Technology Corporation. ThinSOT is a trademark of Linear Technology Corporation. *Patent pending.
APPLICATIO S

Network Servers Wireless Base Stations Optical Networking Systems Mulitvoltage Systems Desktop and Notebook Computers Handheld Devices
TYPICAL APPLICATIO
12V DC/DC DC/DC DC/DC DC/DC DC/DC 3.3V 2.5V
Six Supply Monitor with 5% Tolerance (12V, 3.3V, 2.5V, 1.8V, 1.5V, 1.2V) RST Output Voltage vs V1 with 10k Pull-Up Resistor to V1
0.4 V3 = V4 = VADJ1 = VADJ2 = GND
RST OUTPUT VOLTAGE (V)
1.8V 1.5V 1.2V 0.1F 0.1F 2.15M 100k 124k 100k
SYSTEM
V1
V2 V3 V4 VADJ1 LTC2908-B1 GND
VADJ2 RST
2908 TA01a
U
0.3 0.2 V2 = GND 0.1 V2 = V1 0 0 0.2 0.4 V1 (V)
2908 TA01b
U
U
0.6
0.8
sn2908 2908fs
1
LTC2908
ABSOLUTE AXI U RATI GS (Notes 1, 2)
Storage Temperature Range DFN Package .....................................-65C to 125C TSOT-23 Package ..............................-65C to 150C Lead Temperature (Soldering, 10 sec)................. 300C Terminal Voltages V1, V2, V3, V4 ........................................ - 0.3V to 7V VADJ1, VADJ2 ........................... - 0.3V to (VCC + 0.3V) RST ........................................................ - 0.3V to 7V Operating Temperature Range LTC2908C-A1/LTC2908C-B1 .................. 0C to 70C LTC2908I-A1/LTC2908I-B1 ................ -40C to 85C
PACKAGE/ORDER I FOR ATIO
TOP VIEW GND 1 RST 2 V4 3 V2 4 9 8 7 6 5 VADJ2 V3 VADJ1 V1
ORDER PART NUMBER LTC2908CDDB-A1 LTC2908IDDB-A1 LTC2908CDDB-B1 LTC2908IDDB-B1 DDB8 PART MARKING LBFD LBFF LBFG LBFH
TOP VIEW V2 1 V4 2 RST 3 GND 4 8 V1 7 VADJ1 6 V3 5 VADJ2
DDB8 PACKAGE 8-LEAD (3mm x 2mm) PLASTIC DFN TJMAX = 125C, JA = 76C/ W EXPOSED PAD (PIN 9) (PCB CONNECTION OPTIONAL)
Consult factory for parts specified with wider operating temperature ranges.
ELECTRICAL CHARACTERISTICS
SYMBOL VRT50 VRT33 VRT25 VRT18 VRTADJ PARAMETER 5V, 5% Reset Threshold 3.3V, 5% Reset Threshold 2.5V, 5% Reset Threshold 1.8V, 5% Reset Threshold ADJ, 5% Reset Threshold
(LTC2908-A1) The denotes specifications which apply over the full operating temperature range, otherwise specifications are TA = 25C. VCC = 5V, unless otherwise noted. (Note 2)
CONDITIONS V1 Input Threshold V2 Input Threshold V3 Input Threshold V4 Input Threshold VADJ1, VADJ2 Input Threshold

(LTC2908-B1) The denotes specifications which apply over the full operating temperature range, otherwise specifications are TA = 25C. VCC = 3.3V, unless otherwise noted. (Note 2)
SYMBOL VRT33 VRT25 VRT18 VRT15 VRTADJ PARAMETER 3.3V, 5% Reset Threshold 2.5V, 5% Reset Threshold 1.8V, 5% Reset Threshold 1.5V, 5% Reset Threshold ADJ, 5% Reset Threshold CONDITIONS V1 Input Threshold V2 Input Threshold V3 Input Threshold V4 Input Threshold VADJ1, VADJ2 Input Threshold

2
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U
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WW
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ORDER PART NUMBER LTC2908CTS8-A1 LTC2908ITS8-A1 LTC2908CTS8-B1 LTC2908ITS8-B1 TS8 PART MARKING LTBFJ LTBFK LTBFM LTBFN
TS8 PACKAGE 8-LEAD PLASTIC TSOT-23 TJMAX = 125C, JA = 250C/ W
MIN 4.600 3.036 2.300 1.656 0.492
TYP 4.675 3.086 2.338 1.683 0.500
MAX 4.750 3.135 2.375 1.710 0.508
UNITS V V V V V
MIN 3.036 2.300 1.656 1.380 0.492
TYP 3.086 2.338 1.683 1.403 0.500
MAX 3.135 2.375 1.710 1.425 0.508
UNITS V V V V V
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LTC2908
ELECTRICAL CHARACTERISTICS
SYMBOL VCC IV1 IV2 IV3 IV4 IVADJ tRST tUV VOH VOL PARAMETER Internal Operating Voltage V1 Input Current V2 Input Current V3 Input Current V4 Input Current VADJ1, VADJ2 Input Current Reset Time-Out Period VX Undervoltage Detect to RST or RST Output Voltage High RST (Note 5) Output Voltage Low RST
The denotes specifications which apply over the full operating temperature range, otherwise specifications are TA = 25C. VCC = 5V for the LT2908-A1 and VCC = 3.3V for the LTC2908-B1, unless otherwise noted. (Notes 2, 3)
CONDITIONS RST in Correct Logic State V1 = 5.0V (LTC2908-A1) (Note 4) V1 = 3.3V (LTC2908-B1) V2 = 3.3V (LTC2908-A1) (Note 4) V2 = 2.5V (LTC2908-B1) V3 = 2.5V (LTC2908-A1) V3 = 1.8V (LTC2908-B1) V4 = 1.8V (LTC2908-A1) V4 = 1.5V (LTC2908-B1) VADJ1 = VADJ2 = 0.55V VX Less Than Reset Threshold VRTX by More Than 1% IRST = -1A, VCC = 5V (LTC2908-A1) IRST = -1A, VCC = 3.3V (LTC2908-B1) VCC = 0.5V, IRST = 5A VCC = 1.0V, IRST = 100A VCC = 3.0V, IRST = 2500A

MIN 0.5
TYP 26 24 10 8 2 2 2 2
MAX 6 50 50 20 20 5 5 5 5 15 260
UNITS V A A A A A A A A nA ms s V V
160
200 250
VCC - 1.5 VCC - 1.0 0.01 0.01 0.10 0.15 0.15 0.30
V V V
Note 1: Absolute Maximum Ratings are those values beyond which the life of a device may be impaired. Note 2: The greater of V1, V2 is the internal supply voltage (VCC). Note 3: All currents into pins are positive; all voltages are referenced to GND unless otherwise noted.
Note 4: Under typical operating conditions, most of the quiescent current is drawn from the V1 input. When V2 exceeds V1, V2 supplies most of the quiescent current. Note 5: The output pin RST has an internal pull-up to VCC of typically 6A. However, an external pull-up resistor may be used when a faster rise time is required or for VOH voltages greater than VCC.
TYPICAL PERFOR A CE CHARACTERISTICS
Specifications are at TA = 25C unless otherwise noted. 5V Threshold Voltage vs Temperature
4.750
THRESHOLD VOLTAGE, VRT50 (V) THRESHOLD VOLTAGE, VRT33 (V)
4.725 4.700 4.675 4.650 4.625 4.600 -50
3.115
THRESHOLD VOLTAGE, VRT25 (V)
-25
50 25 0 TEMPERATURE (C)
UW
75
2908 G01
3.3V Threshold Voltage vs Temperature
3.135 2.375
2.5V Threshold Voltage vs Temperature
2.360
3.095
2.345
3.075
2.330
3.055
2.315
100
3.035 -50
-25
50 25 0 TEMPERATURE (C)
75
100
2908 G02
2.300 -50
-25
50 25 0 TEMPERATURE (C)
75
100
2908 G03
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3
LTC2908 TYPICAL PERFOR A CE CHARACTERISTICS
Specifications are at TA = 25C unless otherwise noted. 1.8V Threshold Voltage vs Temperature
1.710
THRESHOLD VOLTAGE, VRT18 (V)
THRESHOLD VOLTAGE, VRT15 (V)
1.700
1.415 1.410 1.405 1.400 1.395 1.390 1.385
THRESHOLD VOLTAGE, VRTADJ (V)
1.690
1.680
1.670
1.660 -50
-25
50 25 0 TEMPERATURE (C)
IV1 vs Temperature
33 V1 = 5.0V (A1)/V1 = 3.3V (B1) 31 V2 = 3.3V (A1)/V2 = 2.5V (B1) V3 = 2.5V (A1)/V3 = 1.8V (B1) 29 V4 = 1.8V (A1)/V4 = 1.5V (B1) VADJ1 = VADJ2 = 0.55V 27 IV1 (A) A1 25 23 21 19 17 -50 -25 0 25 50 TEMPERATURE (C) 75 100
2908 G07
B1
IV2 (A)
9 8 7 6 5 4 -50 -25
A1
B1
IV3 (A)
IV4 vs Temperature
TYPICAL TRANSIENT DURATION (s)
600 500 400 300 200 100 0 0.1 1 10 100 COMPARATOR OVERDRIVE VOLTAGE (% OF VRTX)
2908 G11
RESET TIME-OUT PERIOD, tRST (ms)
V1 = 5.0V (A1)/V1 = 3.3V (B1) 2.1 V2 = 3.3V (A1)/V2 = 2.5V (B1) V3 = 2.5V (A1)/V3 = 1.8V (B1) 2.0 V4 = 1.8V (A1)/V4 = 1.5V (B1) VADJ1 = VADJ2 = 0.55V 1.9
IV4 (A)
2.2
1.8 1.7 1.6 1.5 1.4 -50 -25 50 25 TEMPERATURE (C) 0 75 100
2908 G10
4
UW
75
2908 G04
1.5V Threshold Voltage vs Temperature
1.425 1.420
0.5080 0.5060 0.5040 0.5020 0.5000 0.4980 0.4960 0.4940
ADJ Threshold Voltage vs Temperature
1.380
100
-50
-25
0 25 50 TEMPERATURE (C)
75
100
0.4920 -50
-25
0 50 25 TEMPERATURE (C)
75
100
2908 G06
2908 G05
IV2 vs Temperature
V1 = 5.0V (A1)/V1 = 3.3V (B1) 13 V2 = 3.3V (A1)/V2 = 2.5V (B1) V3 = 2.5V (A1)/V3 = 1.8V (B1) 12 V4 = 1.8V (A1)/V4 = 1.5V (B1) 11 VADJ1 = VADJ2 = 0.55V 10 14
IV3 vs Temperature
V1 = 5.0V (A1)/V1 = 3.3V (B1) 2.1 V2 = 3.3V (A1)/V2 = 2.5V (B1) V3 = 2.5V (A1)/V3 = 1.8V (B1) 2.0 V4 = 1.8V (A1)/V4 = 1.5V (B1) VADJ1 = VADJ2 = 0.55V 1.9 1.8 1.7 1.6 1.5 2.2
50 25 TEMPERATURE (C)
0
75
100
2908 G08
1.4 -50
-25
0 50 25 TEMPERATURE (C)
75
100
2908 G09
Typical Transient Duration vs Comparator Overdrive
700 TA = 25C
250 240 230 220 210 200 190 180 170 160
Reset Time-Out Period (tRST) vs Temperature
RESET OCCURS ABOVE CURVE
150 -50
-25
0 50 25 TEMPERATURE (C)
75
100
2908 G12
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LTC2908 TYPICAL PERFOR A CE CHARACTERISTICS
Specifications are at TA = 25C unless otherwise noted. RST Output Voltage vs V1 with 10k Pull-Up Resistor to V1
5.0 V1 = V2 V3 = 2.5V (A1)/V3 = 1.8V (B1) V4 = 1.8V (A1)/V4 = 1.5V (B1) VADJ1 = VADJ2 = 0.55V 0.4
RST PULL-DOWN CURRENT, IRST (mA)
RST OUTPUT VOLTAGE (V)
RST OUTPUT VOLTAGE (V)
4.0
3.0
2.0
1.0 VRT33 LTC2908-B1 0 0 1 2 V1 (V)
2908 G13
VRT50 LTC2908-A1 4 5 0 0 0.2
3
RST Pull-Down Current (IRST) vs Supply Voltage (VCC) with Dual Channel Supply
10 RST PULL-DOWN CURRENT, IRST (mA) RST PULL-DOWN CURRENT, IRST (mA) VCC = V1 = V2 V3 = V4 = VADJ1 = VADJ2 = GND RST AT 150mV 10
1
1
RST OUTPUT VOLTAGE LOW, VOL (V)
0.1 RST AT 50mV
0.01
0.001 0 0.2 0.4 0.6 0.8 SUPPLY VOLTAGE, VCC (V) 1
2908 G16
RST Output Voltage Low (VOL) vs RST Pull-Down Current (IRST) LTC2908-B1
0.8
RST OUTPUT VOLTAGE LOW, VOL (V)
RST PULL-UP CURRENT, IRST (A)
V1 = 3.3V 0.7 V2 = 2.5V V3 = 1.8V 0.6 V4 = 1.5V VADJ1 = VADJ2 = 0.4V 0.5 85C 0.4 25C 0.3 0.2 0.1 0 0 20 15 RST PULL-DOWN CURRENT, IRST (mA) 5 10 25 -40C
UW
RST Output Voltage vs V1 with 10k Pull-Up Resistor to V1
V3 = V4 = VADJ1 = VADJ2 = GND
6 5 4 3 2 1 0
RST Pull-Down Current (IRST) vs Supply Voltage (VCC)
VCC = V1 = V2 V3 = V4 = VADJ1 = VADJ2 = GND RST AT 150mV
0.3
0.2 V2 = GND 0.1 V2 = V1
RST AT 50mV VRT50 VRT33 LTC2908-B1 LTC2908-A1 0 1 2 3 4 SUPPLY VOLTAGE, VCC (V) 5
2908 G15
0.4 V1 (V)
0.6
0.8
2908 G14
RST Pull-Down Current (IRST) vs Supply Voltage (VCC) with Single Channel Supply
VCC = V1 V2 = V3 = V4 = VADJ1 = VADJ2 = GND
RST Output Voltage Low (VOL) vs RST Pull-Down Current (IRST) LTC2908-A1
0.8 V1 = 5.0V 0.7 V2 = 3.3V V3 = 2.5V 0.6 V4 = 1.8V VADJ1 = VADJ2 = 0.4V 0.5 85C 0.4 25C 0.3 0.2 0.1 0 0 5 10 20 25 30 35 15 RST PULL-DOWN CURRENT, IRST (mA)
2908 G18
RST AT 150mV 0.1
-40C
RST AT 50mV
0.01
0.001 0 0.2 0.4 0.6 0.8 SUPPLY VOLTAGE, VCC (V) 1
2908 G17
RST Pull-Up Current (IRST) vs Supply Voltage (VCC)
-30 -25 -20 -15 -10 -5 VRT33 LTC2908-B1 0 2 2.5 VRT50 LTC2908-A1 5
2908 G20
VCC = V1 = V2 V3 = 2.5V (A1)/V3 = 1.8V (B1) V4 = 1.8V (A1)/V4 = 1.5V (B1) VADJ1 = VADJ2 = 0.55V RST HELD AT 0V
4.5 3 3.5 4 SUPPLY VOLTAGE, VCC (V)
2908 G19
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LTC2908 TYPICAL PERFOR A CE CHARACTERISTICS
RST Output Voltage High (VOH) vs RST Output Source Current (IRST) LTC2908-A1
5
RST OUTPUT VOLTAGE HIGH, VOH (V) RST OUTPUT VOLTAGE HIGH, VOH (V) 3.0
4
3 25C 2 -40C V1 = 5.0V V2 = 3.3V V3 = 2.5V V4 = 1.8V VADJ1 = VADJ2 = 0.55V 0 85C
1
-4 -12 -16 -20 -8 OUTPUT SOURCE CURRENT, IRST (A)
2908 G21
PI FU CTIO S
(TS8 Package/DDB8 Package)
V2 (Pin 1/Pin 4): Voltage Input 2. The greater of V1, V2 is also the internal VCC. The operating voltage on this pin shall not exceed 6V. When in normal operation (V1 > V2), this pin draws approximately 8A. When this pin is acting as the VCC (V2 > V1), this pin draws an additional 15A. Bypass this pin to ground with a 0.1F (or greater) capacitor. V4 (Pin 2/Pin 3): Voltage Input 4. RST (Pin 3/Pin 2): Reset Logic Output. Pulls low when any voltage input is below the reset threshold and is held low for 200ms after all voltage inputs are above threshold. This pin has a weak pull-up to VCC and may be pulled above VCC using an external pull-up. GND (Pin 4/Pin 1): Device Ground.
6
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RST Output Voltage High (VOH) vs RST Output Source Current (IRST) LTC2908-B1
2.5
2.0 25C 85C -40C V1 = 3.3V V2 = 2.5V 1.0 V3 = 1.8V V4 = 1.5V VADJ1 = VADJ2 = 0.55V 0.5 -4 -6 -8 -10 -12 0 -2 OUTPUT SOURCE CURRENT, IRST (A)
2908 G22
1.5
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VADJ2 (Pin 5/Pin 8): Adjustable Voltage Input 2. See Table 1 for recommended ADJ resistors values. V3 (Pin 6/Pin 7): Voltage Input 3. VADJ1 (Pin 7/Pin 6): Adjustable Voltage Input 1. See Table 1 for recommended ADJ resistors values. V1 (Pin 8/Pin 5): Voltage Input 1. The greater of V1, V2 is also the internal VCC. The operating voltage on this pin shall not exceed 6V. When in normal operation (V1 > V2), this pin draws approximately 21A. When this pin is not acting as the VCC (V2 > V1), this pin draws approximately 8A. Bypass this pin to ground with a 0.1F (or greater) capacitor. Exposed Pad (Pin 9, DDB8 Only): Exposed Pad may be left open or connected to device ground.
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LTC2908
BLOCK DIAGRA
V1
POWER DETECT V2
VCC
GND
TI I G DIAGRA
VX
VRTX tUV tRST 1V
2908 TD
RST
+
-
VADJ2
+
-
VADJ1
+
-
V4
+
-
V3
+
+
-
-
W
W
LTC2908-A1/LTC2908-B1 C1 C2 VCC 6A C3 RST 200ms RESET PULSE GENERATOR C4 C5 C6 BANDGAP REFERENCE
2908 BD
UW
VX Monitor Timing
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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 thresholds for the reset not to be invoked. The LTC2908 asserts the reset output during power-up, power-down and brownout 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 pulldown 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 VCC = V1 = V2 V3 = V4 = VADJ1 = VADJ2 = GND RST AT 150mV
RST PULL-DOWN CURRENT, IRST (mA)
1
RST PULL-DOWN CURRENT, IRST (mA)
0.1 RST AT 50mV
0.01
0.001 0 0.4 0.6 0.8 0.2 SUPPLY VOLTAGE, VCC (V) 1
2908 G16
Figure 1. RST Pull-Down Current (IRST) vs Supply Voltage (VCC) 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 characteristic, 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 pulldown 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 (VCC) that powers the other internal circuitry. Once all the VX inputs rise above their thresholds, an internal timer is started. After the internal timer counts a 200ms delay time, RST weakly pulls high to VCC. Power-Down On power-down, once any of the VX inputs drop below their threshold, RST asserts logic low. VCC of at least 0.5V guarantees a logic low of 0.15V at RST.
10 VCC = V1 V2 = V3 = V4 = VADJ1 = VADJ2 = 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, VCC (V) 1
2908 G17
W
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Figure 2. RST Pull-Down Current (IRST) vs Supply Voltage (VCC) with Single Channel Supply
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LTC2908
APPLICATIO S I FOR ATIO
Adjustable Input The noninverting input on the VADJ comparator is set to 0.5V. And the high impedance inverting input directly ties to the VADJ pin. In a typical application, this pin connects to a tap point on an external resistive divider between the positive voltage being monitored and ground. The following formula derives the value of the R1 resistor in the divider from a particular value of R2 and the desired trip voltage: V R1 = TRIP - 1 R2 0.5V R2 = 100k is recommended. Table 1 shows suggested 1% resistor values for various adjustable applications and their corresponding trip thresholds.
Table 1. Suggested 1% Resistor Values for the VADJ Inputs VSUPPLY (V) 12 10 8 7.5 6 5 3.3 3 2.5 1.8 1.5 1.2 1.0 0.9 0.8 0.7 0.6 VTRIP (V) 11.25 9.4 7.5 7 5.6 4.725 3.055 2.82 2.325 1.685 1.410 1.120 0.933 0.840 0.750 0.655 0.561 R1 (k) 2150 1780 1400 1300 1020 845 511 464 365 237 182 124 86.6 68.1 49.9 30.9 12.1 R2 (k) 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100
4.750V 1.5% THRESHOLD 4.675V BAND 4.600V 4.500V
If an application has less than six supply voltages, the unused supervisor inputs should be tied to the closest higher supply voltage available.
U
VTRIP R1 1% VADJ R2 1% LTC2908-A1/LTC2908-B1
W
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- + + -
0.5V
2908 F03
Figure 3. Setting the Adjustable Trip Point
Threshold Accuracy Specifying system voltage margin for worst-case operation requires the consideration of three factors: power supply tolerance, IC supply voltage tolerance and supervisor reset threshold accuracy. Highly accurate supervisors ease the design challenge by decreasing the overall voltage margin required for reliable system operation. Consider a 5V system with a 5% power supply tolerance band. System ICs powered by this supply must operate reliably within this band (and a little more, as explained below). The bottom of the supply tolerance band, at 4.75V (5% below 5V), is the exact voltage at which a perfectly accurate supervisor generates a reset (see Figure 4). Such a perfectly accurate supervisor does not exist--the actual reset threshold may vary over a specified band (1.5% for the LTC2908 supervisors). Figure 5 shows the typical relative threshold accuracy for all six inputs over temperature.
5.000V MINIMUM IDEAL SUPPLY RELIABLE TOLERANCE SUPERVISOR SYSTEM THRESHOLD VOLTAGE NOMINAL SUPPLY VOLTAGE
-5.0% -6.5% REGION OF POTENTIAL MALFUNCTION WITH 2.5% MONITOR
2908 F04
-8.0% -10%
2.5% THRESHOLD BAND
Figure 4. Threshold Band Diagram
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LTC2908
APPLICATIO S I FOR ATIO
1.5 TYPICAL THRESHOLD ACCURACY (%) 1.0 0.5 0 -0.5 -1.0 -1.5 -50
-25
0 25 50 TEMPERATURE (C)
75
100
2908 F05
Figure 5. Typical Threshold Accuracy vs Temperature
With this variation of reset threshold in mind, the nominal reset threshold of the supervisor resides below the minimum supply voltage; just enough so that the reset threshold band and the power supply tolerance bands do not overlap. If the two bands overlap, the supervisor could generate a false or nuisance reset when the power supply remains within its specified tolerance band (for example at 4.8V). Adding half of the reset threshold accuracy spread (1.5%) to the ideal 5% thresholds puts the LTC2908 thresholds at 6.5% (typ) below the nominal input voltage. For example, the 5V typical threshold is 4.675V, or 75mV below the ideal threshold of 4.750V. The guaranteed threshold lies in the band between 4.600V (8% below 5V) and 4.750V (5% below 5V) over temperature. The powered system must work reliably down to the lowest voltage in the threshold band or risk malfunction before the reset line falls. In the 5V example, using the 1.5% accurate supervisor, the system ICs must work down to 4.60V (8% below 5V). System ICs working with a 2.5% accurate supervisor must operate down to 4.50V (10% below 5V), increasing the required system voltage margin and the probability of system malfunction. In any supervisory application, supply noise riding on the monitored DC voltage can cause spurious resets, particularly when the monitored voltage is near the reset threshold. A less desirable but common solution to this problem
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is to introduce hysteresis around the nominal threshold. Notice however, this hysteresis introduces an error term in the threshold accuracy. Therefore, a 2.5% accurate monitor with 1% hysteresis is equivalent to a 3.5% monitor with no hysteresis. Therefore, the LTC2908 takes a different approach to solving this problem of supply noise causing spurious reset. The first line of defense against this spurious reset is a first order lowpass filter at the output of the comparators. Therefore, each comparator output is integrated over time before triggering the output logic. Therefore, any kind of transient at the input of the comparator needs to be of sufficient magnitude and duration before it can trigger a change in the output logic. The second line of defense is the 200ms delay time tRST. This delay eliminates the effect of any supply noise, whose frequency is above 1/200ms = 5Hz, on the RST output. When any one of the supply voltages drops below its threshold, the RST pin asserts low. When the supply recovers above its threshold, the reset-pulse-generator timer starts counting. If all the supplies remain above their corresponding threshold when the timer finishes counting, the RST pin weakly pulls high. However, if any of the supplies falls below its threshold any time during the period when the timer is still counting, the timer resets and it starts fresh when all the supplies rise above their corresponding threshold. Note that this second line of defense is only effective for a rising supply and does not affect the sensitivity of the system to a falling supply. Therefore, the first line of defense that works for both cases of rising and falling is necessary. These two approaches prevent spurious reset caused by supply noise without sacrificing the threshold accuracy. Although all six comparators for the six inputs have builtin glitch filtering, use bypass capacitors on the V1 and V2 inputs because the greater of V1 or V2 supplies the VCC for the part (a 0.1F ceramic capacitor satisfies most applications). Apply filter capacitors on the V3, V4, VADJ1 and VADJ2 inputs in extremely noisy situations.
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LTC2908
APPLICATIO S I FOR ATIO
RST Output Characteristics The DC characteristics of the RST pull-up and pull-down strength are shown in the Typical Performance Characteristics section. The RST output has a weak internal pull-up to VCC = Max(V1, V2) and a strong pull-down to ground. The weak pull-up and strong pull-down arrangement allows this pin to have open-drain behavior while possessing several other beneficial characteristics. The weak pull-up eliminates the need for external pull-up resistors when the rise time on these pins is not critical. On the other hand, the open-drain RST configuration allows for wired-OR connections and can be useful when more than one signal needs to pull down on the RST line. As noted in the discussion of power-up and power-down, the circuits that drive RST are powered by VCC. During fault condition, VCC of at least 0.5V guarantees a maximum VOL = 0.15V at RST. Output Rise and Fall Time Estimation The following formula estimates the output fall time (90% to 10%) for a particular external load capacitance (CLOAD): tFALL 2.2 * RPD * CLOAD where RPD is the on-resistance of the internal pull-down transistor estimated to be typically 40 at room temperature (25C) and CLOAD is the external load capacitance on the pin. Assuming a 150pF load capacitance, the fall time is about 13ns.
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The rise time on the RST pin is limited by a weak internal pull-up current source to VCC. The following formula estimates the output rise time (10% to 90%) at the RST pin: tRISE 2.2 * RPU * CLOAD where RPU is the on-resistance of the pull-up transistor. Notice that this pull-up transistor is modeled as a 6A current source in the Block Diagram as a typical representation. The on-resistance as a function of the VCC = Max(V1, V2) voltage (for VCC > 1V) at room temperature is estimated as follows: RPU = 6 * 105 MAX( V1, V2) - 1V At VCC = 3.3V, RPU is about 260k. Using 150pF for load capacitance, the rise time is 86s. A smaller external pullup resistor may be used if the output needs to pull up faster and/or to a higher voltage. For example, the rise time reduces to 3.3s for a 150pF load capacitance when using a 10k pull-up resistor.
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LTC2908
TYPICAL APPLICATIO S
Six Supply Monitor, 5% Tolerance, 12V, 5V, 3.3V, 2.5V, 1.8V, 1V
12V DC/DC DC/DC DC/DC DC/DC DC/DC 5V 3.3V 2.5V 1.8V 1.0V C1 C2 0.1F 0.1F R1 2.15M R3 R2 100k 86.6k R4 100k SYSTEM
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V1
V2 V3 V4 VADJ1 LTC2908-A1 GND
VADJ2 RST
2908 TA02
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LTC2908
TYPICAL APPLICATIO S
Quad Supply Monitor with One Adjustable Input, 5% Tolerance, 3.3V, 2.5V, 1.8V, 1.2V
3.3V 2.5V SYSTEM DC/DC DC/DC 1.8V 1.2V C1 C2 0.1F 0.1F R3 124k R4 100k
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DC/DC
V1
V2
V3 V4 VADJ1 LTC2908-B1 GND
VADJ2 RST
2908 TA03
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LTC2908
PACKAGE DESCRIPTIO
0.61 0.05 (2 SIDES) 0.675 0.05 2.50 0.05 1.15 0.05 PACKAGE OUTLINE 0.25 0.05 0.50 BSC 2.20 0.05 (2 SIDES) RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS R = 0.115 TYP 5 0.56 0.05 (2 SIDES) 2.00 0.10 (2 SIDES)
PIN 1 BAR TOP MARK (SEE NOTE 6)
0.200 REF
NOTE: 1. DRAWING CONFORMS TO VERSION (WECD-1) IN JEDEC PACKAGE OUTLINE M0-229 2. DRAWING NOT TO SCALE 3. ALL DIMENSIONS ARE IN MILLIMETERS 4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON ANY SIDE 5. EXPOSED PAD SHALL BE SOLDER PLATED 6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION ON THE TOP AND BOTTOM OF PACKAGE
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DDB Package 8-Lead Plastic DFN (3mm x 2mm)
(Reference LTC DWG # 05-08-1702)
3.00 0.10 (2 SIDES) 0.38 0.10 8 0.75 0.05 4 0.25 0.05 2.15 0.05 (2 SIDES) BOTTOM VIEW--EXPOSED PAD 1 PIN 1 CHAMFER OF EXPOSED PAD
(DDB8) DFN 1103
0.50 BSC
0 - 0.05
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LTC2908
PACKAGE DESCRIPTIO
0.52 MAX
3.85 MAX 2.62 REF
RECOMMENDED SOLDER PAD LAYOUT PER IPC CALCULATOR
0.20 BSC 1.00 MAX DATUM `A'
0.30 - 0.50 REF
NOTE: 1. DIMENSIONS ARE IN MILLIMETERS 2. DRAWING NOT TO SCALE 3. DIMENSIONS ARE INCLUSIVE OF PLATING 4. DIMENSIONS ARE EXCLUSIVE OF MOLD FLASH AND METAL BURR 5. MOLD FLASH SHALL NOT EXCEED 0.254mm 6. JEDEC PACKAGE REFERENCE IS MO-193
Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
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TS8 Package 8-Lead Plastic TSOT-23
(Reference LTC DWG # 05-08-1637)
0.65 REF 2.90 BSC (NOTE 4) 1.22 REF 1.4 MIN 2.80 BSC 1.50 - 1.75 (NOTE 4) PIN ONE ID 0.65 BSC 0.22 - 0.36 8 PLCS (NOTE 3) 0.80 - 0.90 0.01 - 0.10 0.09 - 0.20 (NOTE 3) 1.95 BSC
TS8 TSOT-23 0802
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LTC2908
TYPICAL APPLICATIO
Six Supply Monitor with Manual Reset Button, 5% Tolerance, 12V, 5V, 3.3V, 2.5V, 1.8V, 1.5V
12V DC/DC DC/DC DC/DC DC/DC DC/DC 5V 3.3V 2.5V 1.8V 1.5V C1 C2 0.1F 0.1F R1 182k R2 100k R3 MANUAL 2.15M RESET BUTTON R5 (NORMALLY OPEN) 10k R4 100k V1 V2 V3 V4 VADJ1 LTC2908-A1 GND VADJ2 RST
2908 TA04
RELATED PARTS
PART NUMBER LTC690 LTC694-3.3 LTC699 LTC1232 LTC1326/LTC1326-2.5 LTC1536 LTC1726-2.5/LTC1726-5 LTC1727-2.5/LTC1727-5 LTC1728-1.8/LTC1728-3.3 LTC1728-2.5/LTC1728-5 LTC1985-1.8 LTC2900 LTC2901 LTC2902 LTC2903 LTC2904 LTC2905 LTC2906 LTC2907 DESCRIPTION 5V Supply Monitor, Watchdog Timer and Battery Backup 3.3V Supply Monitor, Watchdog Timer and Battery Backup 5V Supply Monitor and Watchdog Timer 5V Supply Monitor, Watchdog Timer and Pushbutton Reset Micropower Precision Triple Supply Monitor for 5V/2.5V, 3.3V and ADJ Precision Triple Supply Monitor for PCI Applications Micropower Triple Supply Monitor for 2.5V/5V, 3.3V and ADJ Micropower Triple Supply Monitor with Open-Drain Reset Micropower Triple Supply Monitor with Open-Drain Reset Micropower Triple Supply Monitor with Open-Drain Reset Micropower Triple Supply Monitor with Push-Pull Reset Output Programmable Quad Supply Monitor Programmable Quad Supply Monitor Programmable Quad Supply Monitor Precision Quad Supply Monitor Three-State Programmable Precision Dual Supply Monitor Three-State Programmable Precision Dual Supply Monitor Dual Supply Monitor with One Pin Selectable Threshold and One Adjustable Input Dual Supply Monitor with One Pin Selectable Threshold and One Adjustable Input COMMENTS 4.65 Threshold 2.9V Threshold 4.65 Threshold 4.37V/4.62V Threshold 4.725V, 3.118V, 1V Threshold (0.75%) Meets PCI tFAIL Timing Specifications Adjustable RESET and Watchdog Time-Outs Individual Monitor Outputs in MSOP 5-Lead SOT-23 Package 5-Lead SOT-23 Package 5-Lead SOT-23 Package Adjustable RESET, 10-Lead MSOP and DFN Packages Adjustable RESET and Watchdog Timer, 16-Lead SSOP Package Adjustable RESET and Tolerance, 16-Lead SSOP Package 6-Lead SOT-23 Package Adjustable Tolerance, 8-Lead SOT-23 and DFN Packages Adjustable RESET and Tolerance, 8-Lead SOT-23 and DFN Packages 0.5V Adjustable Threshold and Three Supply Tolerances, 8-Lead SOT-23 and DFN Packages 0.5V Adjustable Threshold, RESET and Three Supply Tolerances, 8-Lead SOT-23 and DFN Packages
sn2908 2908fs LT/TP 0504 1K * PRINTED IN USA
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Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 FAX: (408) 434-0507
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SYSTEM
www.linear.com
(c) LINEAR TECHNOLOGY CORPORATION 2004

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