8.1 Overview

The LM57 is a precision, dual-output, temperature switch with analog temperature sensor output. The trip temperature (T_TRIP) is selected from 256 possible values by using two external 1% resistors. The V_TEMP class AB analog output provides a voltage that is proportional to temperature. The LM57 includes an internal reference DAC, analog temperature sensor and analog comparator. The reference DAC is connected to one of the comparator inputs. The reference DAC output voltage (V_TRIP) is controlled by the value of resistance applied to the SENSE pins. The resistance value sets one of 16 "logic" levels at the SENSE pins. These "logic" levels are then decoded and applied to the DAC input, thus the actual resistance tolerance does not directly affect the threshold level accuracy. The result of the reference DAC voltage and the temperature sensor output comparison is provided on two output pins T_OVER and T_OVER.

The V_TEMP output has a programmable gain. The output gain has 4 possible settings as described in Figure 12. The gain setting is dependent on the trip point selected by resistance applied to the SENSE pins.

Built-in temperature hysteresis (T_HYST) prevents the digital outputs from oscillating. The T_OVER and T_OVER will activate when the die temperature exceeds T_TRIP and will release when the temperature falls below a temperature equal to T_TRIP minus T_HYST. T_OVER is active-high with a push-pull structure. T_OVER, is active-low with an open-drain structure. There are two different hysteresis options available that are factory preset. The preset hysteresis can be selected by purchasing the proper order number as described in Device Comparison Table.

Driving the TRIP-TEST high will activate the digital outputs. A processor can check the logic level of the T_OVER or T_OVER, confirming that they changed to their active state. This allows for system production testing verification that the comparator and output circuitry are functional after system assembly. When the TRIP-TEST pin is high, the trip-level reference voltage appears at the V_TEMP pin. Tying T_OVER to TRIP-TEST will latch the output after it trips. It can be cleared by forcing TRIP-TEST low or powering off the LM57.

8.2 Functional Block Diagram

8.3 Feature Description

8.3.1 LM57 V_TEMP Temperature-to-Voltage Transfer Function

The value of the R_SENSE resistors select a trip point and a corresponding V_TEMP gain (J2, J3, J4, or J5).The trip point range associated with a given gain is shown in bold green in Table 1. The V_TEMP gain is selected by the R_SENSE resistors. V_TEMP is valid over the entire temperature range. The V_TEMP gain is selected by the R_SENSE resistors. V_TEMP is valid over the entire temperature range.

Figure 12. Temperature Transfer Characteristics

Table 1. LM57 V_TEMP Temperature to Voltage ⁽¹⁾

Temperature (°C)	VTEMP VOLTAGE (mV)
	J2 (-5.166 mV/°C)	J3 (–7.752 mV/°C)	J4 (–10.339 mV/°C)	J5 (–12.924 mV/°C)
–50	1352.56	2028.80	2705.20	3381.40
–49	1347.60	2021.35	2695.26	3368.98
–48	1342.64	2013.90	2685.32	3356.55
–47	1337.67	2006.44	2675.38	3344.12
–46	1332.70	1998.98	2665.43	3331.68
–45	1327.73	1991.52	2655.47	3319.23
–44	1322.76	1984.05	2645.51	3306.78
–43	1317.78	1976.58	2635.54	3294.32
–42	1312.81	1969.11	2625.57	3281.85
–41	1307.82	1961.63	2615.60	3269.38
–40	1302.84	1954.15	2605.62	3256.90
–39	1297.86	1946.66	2595.63	3244.41
–38	1292.87	1939.17	2585.64	3231.92
–37	1287.88	1931.68	2575.64	3219.42
–36	1282.88	1924.18	2565.64	3206.92
–35	1277.89	1916.68	2555.63	3194.41
–34	1272.89	1909.17	2545.62	3181.89
–33	1267.88	1901.66	2535.60	3169.37
–32	1262.88	1894.15	2525.58	3156.84
–31	1257.87	1886.63	2515.56	3144.30
–30	1252.86	1879.11	2505.52	3131.76
–29	1247.85	1871.59	2495.49	3119.21
–28	1242.84	1864.06	2485.44	3106.66
–27	1237.82	1856.53	2475.40	3094.10
–26	1232.80	1848.99	2465.34	3081.53
–25	1227.78	1841.45	2455.29	3068.96
–24	1222.75	1833.91	2445.23	3056.38
–23	1217.73	1826.36	2435.16	3043.79
–22	1212.70	1818.81	2425.09	3031.20
–21	1207.67	1811.26	2415.01	3018.60
–20	1202.63	1803.70	2404.93	3006.00
–19	1197.59	1796.13	2394.84	2993.38
–18	1192.55	1788.57	2384.74	2980.77
–17	1187.51	1781.00	2374.65	2968.14
–16	1182.46	1773.42	2364.54	2955.51
–15	1177.42	1765.85	2354.44	2942.87
–14	1172.37	1758.26	2344.32	2930.23
–13	1167.31	1750.68	2334.20	2917.58
–12	1162.26	1743.09	2324.08	2904.93
–11	1157.20	1735.50	2313.95	2892.26
–10	1152.14	1727.90	2303.82	2879.60
–9	1147.07	1720.30	2293.68	2866.92
–8	1142.01	1712.69	2283.54	2854.24
–7	1136.94	1705.09	2273.39	2841.55
–6	1131.87	1697.47	2263.24	2828.86
–5	1126.79	1689.86	2253.08	2816.16
–4	1121.72	1682.24	2242.91	2803.45
–3	1116.64	1674.61	2232.74	2790.74
–2	1111.56	1666.99	2222.57	2778.02
–1	1106.47	1659.35	2212.39	2765.30
0	1101.39	1651.72	2202.21	2752.57
1	1096.30	1644.08	2192.02	2739.83
2	1091.20	1636.44	2181.82	2727.08
3	1086.11	1628.79	2171.62	2714.33
4	1081.01	1621.14	2161.42	2701.58
5	1075.91	1613.48	2151.21	2688.82
6	1070.81	1605.83	2141.00	2676.05
7	1065.71	1598.16	2130.78	2663.27
8	1060.60	1590.50	2120.55	2650.49
9	1055.49	1582.83	2110.32	2637.70
10	1050.38	1575.15	2100.09	2624.91
11	1045.26	1567.48	2089.85	2612.10
12	1040.14	1559.80	2079.60	2599.30
13	1035.02	1552.11	2069.35	2586.48
14	1029.90	1544.42	2059.10	2573.66
15	1024.77	1536.73	2048.84	2560.84
16	1019.65	1529.03	2038.57	2548.01
17	1014.51	1521.33	2028.30	2535.17
18	1009.38	1513.63	2018.03	2522.32
19	1004.25	1505.92	2007.75	2509.47
20	999.11	1498.21	1997.46	2496.61
21	993.97	1490.49	1987.17	2483.75
22	988.82	1482.77	1976.88	2470.88
23	983.68	1475.05	1966.58	2458.00
24	978.53	1467.32	1956.27	2445.12
25	973.38	1459.59	1945.96	2432.23
26	968.22	1451.86	1935.64	2419.34
27	963.07	1444.12	1925.32	2406.43
28	957.91	1436.38	1915.00	2393.53
29	952.74	1428.63	1904.67	2380.61
30	947.58	1420.88	1894.33	2367.69
31	942.41	1413.13	1883.99	2354.76
32	937.24	1405.37	1873.64	2341.83
33	932.07	1397.61	1863.29	2328.89
34	926.90	1389.84	1852.94	2315.94
35	921.72	1382.07	1842.57	2302.99
36	916.54	1374.30	1832.21	2290.03
37	911.36	1366.52	1821.84	2277.07
38	906.17	1358.74	1811.46	2264.10
39	900.98	1350.96	1801.08	2251.12
40	895.79	1343.17	1790.69	2238.14
41	890.60	1335.38	1780.30	2225.15
42	885.41	1327.58	1769.90	2212.15
43	880.21	1319.78	1759.50	2199.15
44	875.01	1311.98	1749.09	2186.14
45	869.81	1304.17	1738.68	2173.12
46	864.60	1296.36	1728.26	2160.10
47	859.39	1288.54	1717.84	2147.07
48	854.18	1280.72	1707.41	2134.04
49	848.97	1272.90	1696.98	2121.00
50	843.75	1265.07	1686.54	2107.95
51	838.53	1257.24	1676.10	2094.90
52	833.31	1249.41	1665.65	2081.84
53	828.09	1241.57	1655.20	2068.77
54	822.86	1233.73	1644.74	2055.70
55	817.63	1225.88	1634.28	2042.62
56	812.40	1218.03	1623.81	2029.54
57	807.17	1210.18	1613.34	2016.44
58	801.93	1202.32	1602.86	2003.35
59	796.69	1194.46	1592.38	1990.24
60	791.45	1186.60	1581.89	1977.13
61	786.20	1178.73	1571.40	1964.02
62	780.96	1170.86	1560.90	1950.89
63	775.71	1162.98	1550.40	1937.76
64	770.46	1155.10	1539.89	1924.63
65	765.20	1147.22	1529.37	1911.49
66	759.94	1139.33	1518.86	1898.34
67	754.68	1131.44	1508.33	1885.19
68	749.42	1123.54	1497.80	1872.02
69	744.16	1115.64	1487.27	1858.86
70	738.89	1107.74	1476.73	1845.68
71	733.62	1099.83	1466.19	1832.50
72	728.35	1091.92	1455.64	1819.32
73	723.07	1084.01	1445.08	1806.13
74	717.79	1076.09	1434.53	1792.93
75	712.51	1068.17	1423.96	1779.72
76	707.23	1060.24	1413.39	1766.51
77	701.94	1052.31	1402.82	1753.30
78	696.65	1044.38	1392.24	1740.07
79	691.36	1036.44	1381.65	1726.84
80	686.07	1028.50	1371.07	1713.61
81	680.77	1020.55	1360.47	1700.36
82	675.48	1012.60	1349.87	1687.11
83	670.17	1004.65	1339.27	1673.86
84	664.87	996.69	1328.66	1660.60
85	659.56	988.73	1318.04	1647.33
86	654.25	980.77	1307.42	1634.05
87	648.94	972.80	1296.80	1620.77
88	643.63	964.83	1286.17	1607.49
89	638.31	956.85	1275.53	1594.19
90	632.99	948.87	1264.89	1580.89
91	627.67	940.89	1254.25	1567.59
92	622.35	932.90	1243.60	1554.28
93	617.02	924.91	1232.94	1540.96
94	611.69	916.92	1222.28	1527.63
95	606.36	908.92	1211.61	1514.30
96	601.02	900.91	1200.94	1500.97
97	595.69	892.91	1190.27	1487.62
98	590.34	884.90	1179.59	1474.27
99	585.00	876.88	1168.90	1460.92
100	579.66	868.87	1158.21	1447.55
101	574.31	860.84	1147.52	1434.18
102	568.96	852.82	1136.81	1420.81
103	563.61	844.79	1126.11	1407.43
104	558.25	836.76	1115.40	1394.04
105	552.89	828.72	1104.68	1380.65
106	547.53	820.68	1093.96	1367.24
107	542.17	812.63	1083.23	1353.84
108	536.80	804.59	1072.50	1340.42
109	531.43	796.53	1061.77	1327.01
110	526.06	788.48	1051.02	1313.58
111	520.69	780.42	1040.28	1300.15
112	515.31	772.35	1029.53	1286.71
113	509.93	764.29	1018.77	1273.26
114	504.55	756.21	1008.01	1259.81
115	499.17	748.14	997.24	1246.36
116	493.78	740.06	986.47	1232.89
117	488.39	731.98	975.69	1219.42
118	483.00	723.89	964.91	1205.95
119	477.61	715.80	954.12	1192.46
120	472.21	707.70	943.33	1178.98
121	466.81	699.61	932.53	1165.48
122	461.41	691.50	921.73	1151.98
123	456.00	683.40	910.92	1138.47
124	450.60	675.29	900.11	1124.96
125	445.19	667.18	889.29	1111.44
126	439.78	659.06	878.47	1097.91
127	434.36	650.94	867.64	1084.38
128	428.94	642.81	856.81	1070.84
129	423.52	634.68	845.97	1057.29
130	418.10	626.55	835.13	1043.74
131	412.67	618.41	824.28	1030.18
132	407.25	610.27	813.43	1016.62
133	401.82	602.13	802.57	1003.05
134	396.38	593.98	791.71	989.47
135	390.95	585.83	780.84	975.89
136	385.51	577.67	769.97	962.30
137	380.07	569.51	759.09	948.70
138	374.63	561.35	748.20	935.10
139	369.18	553.18	737.32	921.49
140	363.73	545.01	726.42	907.87
141	358.28	536.84	715.52	894.25
142	352.83	528.66	704.62	880.62
143	347.37	520.48	693.71	866.99
144	341.91	512.29	682.80	853.35
145	336.45	504.10	671.88	839.70
146	330.99	495.91	660.95	826.05
147	325.52	487.71	650.03	812.39
148	320.05	479.51	639.09	798.73
149	314.58	471.30	628.15	785.05
150	309.10	463.09	617.21	771.38

(1) The R_SENSE resistors select a trip point and a corresponding V_TEMP gain (J2, J3, J4, or J5). The trip point range associated with a given gain is shown in bold green on this table. V_TEMP is valid over the entire temperature range.

8.3.1.1 LM57 V_TEMP Voltage-to-Temperature Equations

Equation 1. V_TEMP= a (T-30)²+ b (T-30) + c

where

• V_TEMP is in mV and T is in °C

Equation 2.

where

• T is in °C and V_TEMP is in mV

Table 2. LM57 V_TEMP Voltage-to-Temperature Equations Coefficients

Trip-Point Region	LM57 Trip Point Range	a	b	c
J2	−41°C to 52°C	– 0.00129	− 5.166	947.6
J3	52°C to 97°C	– 0.00191	− 7.752	1420.9
J4	97°C to 119°C	– 0.00253	− 10.339	1894.3
J5	119°C to 150°C	– 0.00316	− 12.924	2367.7

8.3.6 V_TEMP Analog Temperature Sensor Output

The V_TEMP push-pull output provides the ability to sink and source significant current. This is beneficial when, for example, driving dynamic loads like an input stage on an analog-to-digital converter (ADC). In these applications the source current is required to quickly charge the input capacitor of the ADC. See the Typical Application section for more discussion of this topic. The LM57 is ideal for this and other applications which require strong source or sink current.

8.3.6.1 V_TEMP Noise Considerations

A load capacitor on V_TEMP can help to filter noise.

For noisy environments, TI recommends a 100 nF supply decoupling capacitor placed closed across V_DD and GND pins of LM57.

8.3.6.2 V_TEMP Capacitive Loads

The V_TEMP Output handles capacitive loading well. In an extremely noisy environment, or when driving a switched sampling input on an ADC, it may be necessary to add some filtering to minimize noise coupling. Without any precautions, the V_TEMP can drive a capacitive load less than or equal to 1100 pF as shown in Figure 13. For capacitive loads greater than 1100 pF, a series resistor is required on the output, as shown in Figure 14, to maintain stable conditions.

Figure 13. LM57 With No Isolation Resistor Required

Figure 14. LM57 With Series Resistor for Capacitive Loading Greater than 1100 pF

Table 6. C_LOAD and R_S Values of Figure 14

CLOAD	Minimum RS
1.1 to 99 nF	3 kΩ
100 to 999 nF	1.5 kΩ
1 μF	750 Ω

8.3.6.3 V_TEMP Voltage Shift

The LM57 is very linear over temperature and supply voltage range. Due to the intrinsic behavior of an NMOS/PMOS rail-to-rail buffer, a slight shift in the output can occur when the supply voltage is ramped over the operating range of the device. The location of the shift is determined by the relative levels of V_DD and V_TEMP. The shift typically occurs when V_DD − V_TEMP = 1 V.

This slight shift (a few millivolts) takes place over a wide change (approximately 200 mV) in V_DD or V_TEMP. Since the shift takes place over a wide temperature change of 5°C to 20°C, V_TEMP is always monotonic. The accuracy specifications in the table already includes this possible shift.

8.4 Device Functional Modes

The LM57 has several modes of operation as detailed in the following drawings.

Figure 15. Temperature Switch Using Push-Pull Output

Figure 16. Temperature Switch Using Open-Drain Output

As shown in Figure 17 the LM57 has a TRIP Test input simplifying in situ board conductivity testing. Forcing TRIP TEST pin "HIGH" will drive the T_OVER pin "LOW" and the T_OVER pin "HIGH".

Figure 17. Trip Test Digital Output Test Circuit

In the circuit shown in Figure 18 when T_OVER goes active high, it drives trip test high. Trip test high causes T_OVER to stay high. It is therefore latched. To release the latch, power down, then power up. The LM57 always comes up in a released condition.

Figure 18. Simple Latch Circuit

The TRIP TEST pin, normally used to check the operation of the T_OVER and T_OVER pins, may be used to latch the outputs whenever the temperature exceeds the programmed limit and causes the digital outputs to assert. As shown in Figure 19, when T_OVER goes high, the TRIP TEST input is also pulled high and causes T_OVER output to latch high and the T_OVER output to latch low. Momentarily switching the TRIP TEST input low will reset the LM57 to normal operation. The resistor limits the current out of the T_OVER output pin.

Figure 19. Latch Circuit Using T_OVER Output