LTC2908 Precision Six Supply Monitor U FEATURES ■ ■ ■ ■ ■ ■ ■ ■ DESCRIPTIO The LTC®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. 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 × 3mm) DFN Packages The LTC2908 features a tight 1.5% threshold accuracy over the entire operating temperature range (– 40°C to 85°C) 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. U APPLICATIO S ■ ■ ■ ■ ■ Network Servers Wireless Base Stations Optical Networking Systems Mulitvoltage Systems Desktop and Notebook Computers Handheld Devices , LTC and LT are registered trademarks of Linear Technology Corporation. ThinSOT is a trademark of Linear Technology Corporation. *Patent pending. U ■ TYPICAL APPLICATIO 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 12V DC/DC DC/DC DC/DC DC/DC 3.3V 0.4 V3 = V4 = VADJ1 = VADJ2 = GND 2.5V SYSTEM 1.8V 1.5V 1.2V 0.1µF 0.1µF 2.15M 100k 124k RST OUTPUT VOLTAGE (V) DC/DC 0.3 0.2 V2 = GND 0.1 100k V2 = V1 0 V1 V2 V3 V4 VADJ1 LTC2908-B1 GND VADJ2 0 0.2 0.4 0.6 0.8 V1 (V) RST 2908 TA01a 2908 TA01b sn2908 2908fs 1 LTC2908 W W W AXI U U ABSOLUTE RATI GS (Notes 1, 2) 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 .................. 0°C to 70°C LTC2908I-A1/LTC2908I-B1 ................ –40°C to 85°C Storage Temperature Range DFN Package .....................................–65°C to 125°C TSOT-23 Package ..............................–65°C to 150°C Lead Temperature (Soldering, 10 sec)................. 300°C U U W PACKAGE/ORDER I FOR ATIO ORDER PART NUMBER ORDER PART NUMBER TOP VIEW GND 1 8 VADJ2 RST 2 7 V3 6 VADJ1 5 V1 V4 3 V2 4 9 DDB8 PACKAGE 8-LEAD (3mm × 2mm) PLASTIC DFN TJMAX = 125°C, θJA = 76°C/ W EXPOSED PAD (PIN 9) (PCB CONNECTION OPTIONAL) 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 LTC2908CTS8-A1 LTC2908ITS8-A1 LTC2908CTS8-B1 LTC2908ITS8-B1 TS8 PART MARKING TS8 PACKAGE 8-LEAD PLASTIC TSOT-23 TJMAX = 125°C, θJA = 250°C/ W LTBFJ LTBFK LTBFM LTBFN Consult factory for parts specified with wider operating temperature ranges. ELECTRICAL CHARACTERISTICS (LTC2908-A1) The ● denotes specifications which apply over the full operating temperature range, otherwise specifications are TA = 25°C. VCC = 5V, unless otherwise noted. (Note 2) SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS VRT50 5V, 5% Reset Threshold V1 Input Threshold ● 4.600 4.675 4.750 V VRT33 3.3V, 5% Reset Threshold V2 Input Threshold ● 3.036 3.086 3.135 V VRT25 2.5V, 5% Reset Threshold V3 Input Threshold ● 2.300 2.338 2.375 V VRT18 1.8V, 5% Reset Threshold V4 Input Threshold ● 1.656 1.683 1.710 V VRTADJ ADJ, 5% Reset Threshold VADJ1, VADJ2 Input Threshold ● 0.492 0.500 0.508 V (LTC2908-B1) The ● denotes specifications which apply over the full operating temperature range, otherwise specifications are TA = 25°C. VCC = 3.3V, unless otherwise noted. (Note 2) SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS VRT33 3.3V, 5% Reset Threshold V1 Input Threshold ● 3.036 3.086 3.135 V VRT25 2.5V, 5% Reset Threshold VRT18 1.8V, 5% Reset Threshold V2 Input Threshold ● 2.300 2.338 2.375 V V3 Input Threshold ● 1.656 1.683 1.710 V VRT15 1.5V, 5% Reset Threshold V4 Input Threshold ● 1.380 1.403 1.425 V VRTADJ ADJ, 5% Reset Threshold VADJ1, VADJ2 Input Threshold ● 0.492 0.500 0.508 V sn2908 2908fs 2 LTC2908 ELECTRICAL CHARACTERISTICS The ● denotes specifications which apply over the full operating temperature range, otherwise specifications are TA = 25°C. VCC = 5V for the LT2908-A1 and VCC = 3.3V for the LTC2908-B1, unless otherwise noted. (Notes 2, 3) SYMBOL PARAMETER CONDITIONS MIN VCC Internal Operating Voltage RST in Correct Logic State ● IV1 V1 Input Current V1 = 5.0V (LTC2908-A1) (Note 4) V1 = 3.3V (LTC2908-B1) ● ● IV2 V2 Input Current V2 = 3.3V (LTC2908-A1) (Note 4) V2 = 2.5V (LTC2908-B1) IV3 V3 Input Current IV4 TYP 0.5 MAX UNITS 6 V 26 24 50 50 µA µA ● ● 10 8 20 20 µA µA V3 = 2.5V (LTC2908-A1) V3 = 1.8V (LTC2908-B1) ● ● 2 2 5 5 µA µA V4 Input Current V4 = 1.8V (LTC2908-A1) V4 = 1.5V (LTC2908-B1) ● ● 2 2 5 5 µA µA IVADJ VADJ1, VADJ2 Input Current VADJ1 = VADJ2 = 0.55V ● ±15 nA tRST Reset Time-Out Period 260 ms tUV VX Undervoltage Detect to RST or RST VX Less Than Reset Threshold VRTX by More Than 1% VOH Output Voltage High RST (Note 5) IRST = –1µA, VCC = 5V (LTC2908-A1) IRST = –1µA, VCC = 3.3V (LTC2908-B1) ● ● VOL Output Voltage Low RST VCC = 0.5V, IRST = 5µA VCC = 1.0V, IRST = 100µA VCC = 3.0V, IRST = 2500µA ● ● ● ● 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. 160 200 250 µs VCC – 1.5 VCC – 1.0 V V 0.01 0.01 0.10 0.15 0.15 0.30 V V V 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 6µA. However, an external pull-up resistor may be used when a faster rise time is required or for VOH voltages greater than VCC. U W TYPICAL PERFOR A CE CHARACTERISTICS Specifications are at TA = 25°C unless otherwise noted. 5V Threshold Voltage vs Temperature 3.3V Threshold Voltage vs Temperature 4.725 4.700 4.675 4.650 4.625 4.600 –50 –25 50 25 0 TEMPERATURE (°C) 75 100 2908 G01 2.375 THRESHOLD VOLTAGE, VRT25 (V) 3.135 THRESHOLD VOLTAGE, VRT33 (V) THRESHOLD VOLTAGE, VRT50 (V) 4.750 2.5V Threshold Voltage vs Temperature 3.115 3.095 3.075 3.055 3.035 –50 –25 50 25 0 TEMPERATURE (°C) 75 100 2908 G02 2.360 2.345 2.330 2.315 2.300 –50 –25 50 25 0 TEMPERATURE (°C) 75 100 2908 G03 sn2908 2908fs 3 LTC2908 U W TYPICAL PERFOR A CE CHARACTERISTICS Specifications are at TA = 25°C unless otherwise noted. 1.8V Threshold Voltage vs Temperature 1.5V Threshold Voltage vs Temperature 1.700 1.690 1.680 1.670 1.425 0.5080 1.420 0.5060 THRESHOLD VOLTAGE, VRTADJ (V) THRESHOLD VOLTAGE, VRT15 (V) THRESHOLD VOLTAGE, VRT18 (V) 1.710 ADJ Threshold Voltage vs Temperature 1.415 1.410 1.405 1.400 1.395 1.390 1.385 1.380 –25 50 25 0 TEMPERATURE (°C) 75 100 –50 –25 0 25 50 TEMPERATURE (°C) 75 IV1 vs Temperature 0.4980 0.4960 0.4940 0.4920 –50 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 75 IV2 (µA) 27 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 10 A1 9 B1 8 23 1.8 1.7 7 21 100 IV3 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 25 0 50 25 TEMPERATURE (°C) 2.2 14 B1 –25 2908 G06 IV2 vs Temperature 33 IV1 (µA) 0.5000 2908 G05 2908 G04 A1 100 0.5020 IV3 (µA) 1.660 –50 0.5040 1.6 6 19 1.5 5 –25 0 25 50 TEMPERATURE (°C) 75 4 –50 100 –25 0 50 25 TEMPERATURE (°C) 75 2908 G07 700 2.2 TYPICAL TRANSIENT DURATION (µs) IV4 (µA) 1.9 1.8 1.7 1.6 1.5 –25 0 50 25 TEMPERATURE (°C) 75 100 2908 G10 0 50 25 TEMPERATURE (°C) 75 Reset Time-Out Period (tRST) vs Temperature 250 TA = 25°C 600 500 100 2908 G09 Typical Transient Duration vs Comparator Overdrive 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 –25 2908 G08 IV4 vs Temperature 1.4 –50 1.4 –50 100 RESET TIME-OUT PERIOD, tRST (ms) 17 –50 RESET OCCURS ABOVE CURVE 400 300 200 100 0 0.1 1 10 100 COMPARATOR OVERDRIVE VOLTAGE (% OF VRTX) 2908 G11 240 230 220 210 200 190 180 170 160 150 –50 –25 0 50 25 TEMPERATURE (°C) 75 100 2908 G12 sn2908 2908fs 4 LTC2908 U W TYPICAL PERFOR A CE CHARACTERISTICS Specifications are at TA = 25°C unless otherwise noted. RST Output Voltage vs V1 with 10k Pull-Up Resistor to V1 RST OUTPUT VOLTAGE (V) 4.0 RST OUTPUT VOLTAGE (V) 0.4 V1 = V2 V3 = 2.5V (A1)/V3 = 1.8V (B1) V4 = 1.8V (A1)/V4 = 1.5V (B1) VADJ1 = VADJ2 = 0.55V 3.0 2.0 1.0 VRT33 LTC2908-B1 0 0 1 4 6 V3 = V4 = VADJ1 = VADJ2 = GND 0.3 0.2 V2 = GND 0.1 V2 = V1 VRT50 LTC2908-A1 3 2 0 5 0.2 0.4 10 RST AT 50mV VRT50 VRT33 LTC2908-B1 LTC2908-A1 1 0 0.1 RST AT 50mV 0.01 RST Output Voltage Low (VOL) vs RST Pull-Down Current (IRST) LTC2908-A1 0.8 VCC = V1 V2 = V3 = V4 = VADJ1 = VADJ2 = GND 1 RST AT 150mV 0.1 RST AT 50mV 0.01 0.001 0 1 0.2 0.4 0.6 0.8 SUPPLY VOLTAGE, VCC (V) 2908 G16 –30 RST PULL-UP CURRENT, IRST (µA) V1 = 3.3V 0.7 V2 = 2.5V V3 = 1.8V 0.6 V4 = 1.5V VADJ1 = VADJ2 = 0.4V 0.5 –40°C 0.4 25°C 0.3 0.2 0.1 0 10 85°C –40°C 0.4 25°C 0.3 0.2 0.1 0 1 5 10 20 25 30 35 15 RST PULL-DOWN CURRENT, IRST (mA) 20 15 RST PULL-DOWN CURRENT, IRST (mA) 2908 G18 RST Pull-Up Current (IRST) vs Supply Voltage (VCC) 0.8 5 0.5 2908 G17 RST Output Voltage Low (VOL) vs RST Pull-Down Current (IRST) LTC2908-B1 85°C V1 = 5.0V 0.7 V2 = 3.3V V3 = 2.5V 0.6 V4 = 1.8V VADJ1 = VADJ2 = 0.4V 0 0.001 0.2 0.4 0.6 0.8 SUPPLY VOLTAGE, VCC (V) 5 1 2 3 4 SUPPLY VOLTAGE, VCC (V) 2908 G15 RST OUTPUT VOLTAGE LOW, VOL (V) RST AT 150mV RST OUTPUT VOLTAGE LOW, VOL (V) 2 2908 G14 RST PULL-DOWN CURRENT, IRST (mA) RST PULL-DOWN CURRENT, IRST (mA) 3 RST Pull-Down Current (IRST) vs Supply Voltage (VCC) with Single Channel Supply VCC = V1 = V2 V3 = V4 = VADJ1 = VADJ2 = GND 0 RST AT 150mV 4 V1 (V) RST Pull-Down Current (IRST) vs Supply Voltage (VCC) with Dual Channel Supply 0 5 0.8 0.6 2908 G13 1 VCC = V1 = V2 V3 = V4 = VADJ1 = VADJ2 = GND 0 0 V1 (V) 10 RST Pull-Down Current (IRST) vs Supply Voltage (VCC) RST PULL-DOWN CURRENT, IRST (mA) 5.0 RST Output Voltage vs V1 with 10k Pull-Up Resistor to V1 25 2908 G19 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 –25 –20 –15 –10 –5 VRT33 LTC2908-B1 0 2 2.5 VRT50 LTC2908-A1 4.5 3 3.5 4 SUPPLY VOLTAGE, VCC (V) 5 2908 G20 sn2908 2908fs 5 LTC2908 U W TYPICAL PERFOR A CE CHARACTERISTICS RST Output Voltage High (VOH) vs RST Output Source Current (IRST) LTC2908-A1 RST Output Voltage High (VOH) vs RST Output Source Current (IRST) LTC2908-B1 3.0 4 3 25°C –40°C V1 = 5.0V V2 = 3.3V V3 = 2.5V V4 = 1.8V VADJ1 = VADJ2 = 0.55V 2 1 0 85°C –4 –12 –16 –20 –8 OUTPUT SOURCE CURRENT, IRST (µA) RST OUTPUT VOLTAGE HIGH, VOH (V) RST OUTPUT VOLTAGE HIGH, VOH (V) 5 2.5 2.0 25°C 1.5 85°C –40°C 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 (µA) 2908 G22 2908 G21 U U U 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 8µA. When this pin is acting as the VCC (V2 > V1), this pin draws an additional 15µA. Bypass this pin to ground with a 0.1µF (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. 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 21µA. When this pin is not acting as the VCC (V2 > V1), this pin draws approximately 8µA. Bypass this pin to ground with a 0.1µF (or greater) capacitor. Exposed Pad (Pin 9, DDB8 Only): Exposed Pad may be left open or connected to device ground. sn2908 2908fs 6 LTC2908 W BLOCK DIAGRA LTC2908-A1/LTC2908-B1 – V1 POWER DETECT + C1 VCC – V2 + C2 VCC – V3 + 6µA C3 RST 200ms RESET PULSE GENERATOR – V4 + C4 – VADJ1 + C5 – VADJ2 + GND C6 BANDGAP REFERENCE 2908 BD WU W TI I G DIAGRA VX Monitor Timing VX VRTX tUV RST tRST 1V 2908 TD sn2908 2908fs 7 LTC2908 U W U U 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. 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 Power-Up RST PULL-DOWN CURRENT, IRST (mA) RST PULL-DOWN CURRENT, IRST (mA) 10 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. RST AT 150mV 0.1 RST AT 50mV 0.01 VCC = V1 V2 = V3 = V4 = VADJ1 = VADJ2 = GND 1 RST AT 150mV 0.1 RST AT 50mV 0.01 0.001 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 0 0.4 0.6 0.8 0.2 SUPPLY VOLTAGE, VCC (V) 1 2908 G17 Figure 2. RST Pull-Down Current (IRST) vs Supply Voltage (VCC) with Single Channel Supply sn2908 2908fs 8 LTC2908 U W U U APPLICATIO S I FOR ATIO Adjustable Input VTRIP The noninverting input on the VADJ comparator is set to 0.5V. And the high impedance inverting input directly ties to the VADJ pin. LTC2908-A1/LTC2908-B1 R1 1% – VADJ R2 1% + 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: + – 0.5V 2908 F03 Figure 3. Setting the Adjustable Trip Point ⎛V ⎞ R1 = ⎜ TRIP – 1⎟ R2 ⎝ 0.5V ⎠ Threshold Accuracy 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) VTRIP (V) R1 (kΩ) R2 (kΩ) 12 11.25 2150 100 10 9.4 1780 100 8 7.5 1400 100 7.5 7 1300 100 6 5.6 1020 100 5 4.725 845 100 3.3 3.055 511 100 3 2.82 464 100 2.5 2.325 365 100 1.8 1.685 237 100 1.5 1.410 182 100 1.2 1.120 124 100 1.0 0.933 86.6 100 0.9 0.840 68.1 100 0.8 0.750 49.9 100 0.7 0.655 30.9 100 0.6 0.561 12.1 100 If an application has less than six supply voltages, the unused supervisor inputs should be tied to the closest higher supply voltage available. 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 4.750V ±1.5% THRESHOLD 4.675V BAND 4.600V 4.500V 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% ±2.5% THRESHOLD BAND –10% Figure 4. Threshold Band Diagram sn2908 2908fs 9 LTC2908 U W U U APPLICATIO S I FOR ATIO 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. TYPICAL THRESHOLD ACCURACY (%) 1.5 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 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. 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). 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. 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. 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. 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 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. 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 0.1µF ceramic capacitor satisfies most applications). Apply filter capacitors on the V3, V4, VADJ1 and VADJ2 inputs in extremely noisy situations. sn2908 2908fs 10 LTC2908 U W U U 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): 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 6µA 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 Ω MAX( V1, V2) – 1V At VCC = 3.3V, RPU is about 260k. Using 150pF for load capacitance, the rise time is 86µs. 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 3.3µs for a 150pF load capacitance when using a 10k pull-up resistor. tFALL ≈ 2.2 • RPD • CLOAD where RPD is the on-resistance of the internal pull-down transistor estimated to be typically 40Ω at room temperature (25°C) and CLOAD is the external load capacitance on the pin. Assuming a 150pF load capacitance, the fall time is about 13ns. sn2908 2908fs 11 LTC2908 U 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 SYSTEM 2.5V 1.8V 1.0V C1 C2 0.1µF 0.1µF V1 R1 2.15M R3 R2 100k 86.6k V2 V3 V4 VADJ1 LTC2908-A1 GND R4 100k VADJ2 RST 2908 TA02 sn2908 2908fs 12 LTC2908 U TYPICAL APPLICATIO S Quad Supply Monitor with One Adjustable Input, 5% Tolerance, 3.3V, 2.5V, 1.8V, 1.2V 3.3V DC/DC 2.5V SYSTEM DC/DC DC/DC 1.8V 1.2V C1 C2 0.1µF 0.1µF V1 V2 R3 124k V3 V4 VADJ1 LTC2908-B1 GND R4 100k VADJ2 RST 2908 TA03 sn2908 2908fs 13 LTC2908 U PACKAGE DESCRIPTIO DDB Package 8-Lead Plastic DFN (3mm × 2mm) (Reference LTC DWG # 05-08-1702) 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) 3.00 ±0.10 (2 SIDES) PIN 1 BAR TOP MARK (SEE NOTE 6) 0.200 REF 0.38 ± 0.10 8 2.00 ±0.10 (2 SIDES) 0.75 ±0.05 0 – 0.05 4 0.25 ± 0.05 1 PIN 1 CHAMFER OF EXPOSED PAD (DDB8) DFN 1103 0.50 BSC 2.15 ±0.05 (2 SIDES) BOTTOM VIEW—EXPOSED PAD 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 sn2908 2908fs 14 LTC2908 U PACKAGE DESCRIPTIO TS8 Package 8-Lead Plastic TSOT-23 (Reference LTC DWG # 05-08-1637) 0.52 MAX 2.90 BSC (NOTE 4) 0.65 REF 1.22 REF 1.4 MIN 3.85 MAX 2.62 REF 2.80 BSC 1.50 – 1.75 (NOTE 4) PIN ONE ID RECOMMENDED SOLDER PAD LAYOUT PER IPC CALCULATOR 0.22 – 0.36 8 PLCS (NOTE 3) 0.65 BSC 0.80 – 0.90 0.20 BSC 0.01 – 0.10 1.00 MAX DATUM ‘A’ 0.30 – 0.50 REF 0.09 – 0.20 (NOTE 3) 1.95 BSC TS8 TSOT-23 0802 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 sn2908 2908fs 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. 15 LTC2908 U 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 SYSTEM 2.5V 1.8V 1.5V C1 C2 0.1µF 0.1µF R1 182k R3 MANUAL 2.15M RESET BUTTON R5 (NORMALLY OPEN) 10k R2 100k 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 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 LTC2901 Programmable Quad Supply Monitor LTC2902 Programmable Quad Supply Monitor LTC2903 LTC2904 Precision Quad Supply Monitor Three-State Programmable Precision Dual Supply Monitor LTC2905 Three-State Programmable Precision Dual Supply Monitor LTC2906 Dual Supply Monitor with One Pin Selectable Threshold and One Adjustable Input Dual Supply Monitor with One Pin Selectable Threshold and One Adjustable Input LTC2907 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 16 Linear Technology Corporation LT/TP 0504 1K • PRINTED IN USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com © LINEAR TECHNOLOGY CORPORATION 2004