Datasheet MAX917, MAX918, MAX919, MAX920 (Maxim) - 10

SOT23, 1.8V, Nanopower, Beyond-the-Rails Comparators With/Without Reference Output Stage Circuitry
The MAX917–MAX920 contain a unique break-before- VCC make output stage capable of rail-to-rail operation with up to ±8mA loads. Many comparators consume orders of magnitude more current during switching than dur- 120nA ing steady-state operation. However, with this family of comparators, the supply-current change during an out- REF put transition is extremely small. In the Typical Oper- ating Characteristics, the Supply Current vs. Output Transition Frequency graphs show the minimal supply- current increase as the output switching frequency approaches 1kHz. This characteristic reduces the need VEE for power-supply filter capacitors to reduce glitches created by comparator switching currents. In battery- powered applications, this characteristic results in a Figure 1. MAX917/MAX918 Voltage Reference Output substantial increase in battery life. Equivalent Circuit
MAX917–MAX920 Reference (MAX917/MAX918) Internal Hysteresis
The internal reference in the MAX917/MAX918 has an Many comparators oscillate in the linear region of oper- output voltage of +1.245V with respect to VEE. Its typi- ation because of noise or undesired parasitic feed- cal temperature coefficient is 95ppm/°C over the full back. This tends to occur when the voltage on one -40°C to +85°C temperature range. The reference is a input is equal or very close to the voltage on the other PNP emitter-follower driven by a 120nA current source input. The MAX917–MAX920 have internal hysteresis to (Figure 1). The output impedance of the voltage refer- counter parasitic effects and noise. ence is typically 200kΩ, preventing the reference from driving large loads. The reference can be bypassed The hysteresis in a comparator creates two trip points: with a low-leakage capacitor. The reference is stable one for the rising input voltage (VTHR) and one for the for any capacitive load. For applications requiring a falling input voltage (VTHF) (Figure 2). The difference lower output impedance, buffer the reference with a between the trip points is the hysteresis (VHB). When low-input-leakage op amp, such as the MAX406. the comparator’s input voltages are equal, the hystere- sis effectively causes one comparator input to move
Applications Information
quickly past the other, thus taking the input out of the region where oscillation occurs. Figure 2 illustrates the
Low-Voltage, Low-Power Operation
case in which IN- has a fixed voltage applied, and IN+ The MAX917–MAX920 are ideally suited for use with most is varied. If the inputs were reversed, the figure would battery-powered systems. Table 1 lists a variety of battery be the same, except with an inverted output. types, capacities, and approximate operating times for the MAX917–MAX920, assuming nominal conditions.
Table 1. Battery Applications Using MAX917–MAX920 CAPACITY, MAX917/MAX918 MAX919/MAX920 BATTERY VFRESH VEND-OF-LIFE RECHARGEABLE AA SIZE OPERATING TIME OPERATING TIME TYPE (V) (V) (mA-h) (hr) (hr)
Alkaline No 3.0 1.8 2000 2.5 x 106 5 x 106 (2 Cells) Nickel-Cadmium Yes 2.4 1.8 750 937,500 1.875 x 106 (2 Cells) Lithium-Ion Yes 3.5 2.7 1000 1.25 x 106 2.5 x 106 (1 Cell) Nickel-Metal- Hydride Yes 2.4 1.8 1000 1.25 x 106 2.5 x 106 (2 Cells)
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