LINEAR LTC2938 User Guide

design features

4- and 6-Supply Monitors Feature ±1.5% Accuracy and Watchdog Timers for Rails Down to 1.2V A. Ng

Two new power supply monitors from Linear Technology, the LTC2938 and LTC2939, are specifically designed to monitor lower supply voltages (down to 1.2V) in multivoltage systems. The LTC2938 and LTC2939 share the same architecture and differ only in the number of voltages monitored. The LTC2938 is a 4-supply monitor and comes in compact 12-pin MSOP and DFN packages. The LTC2939 monitors six supplies and is offered in a 16-pin MSOP package. Both monitors have a tight threshold accuracy of 1.5% over the operating temperature range, which eases the voltage headroom requirements of circuits powered by the monitored supplies and is much tighter than supply monitors from other manufacturers. Neither monitor requires external calibration or trimming. Both parts are designed for systems with 5% power supply tolerance.

The watchdog circuit in these moni- SINGLE PIN SELECTS FROM The LTC2938 and LTC2939 supply thresh- 16 POSSIBLE THRESHOLD

tors includes a watchdog input (WDI) old voltages are configured by an exter-

COMBINATIONS

and a watchdog output (WDO), which nal resistive divider from the VREF pin

A single pin (VPG) allows the selection of

facilitates microprocessor monitoring to ground (see Figure 2). The center

one of 16 possible threshold configura-

and control. The WDO output is latched tap of the divider drives the VPG pin.

tions. This programmability eliminates

low in the event of a watchdog timeout During power-up, the voltage at the

the need to qualify, source and stock

and allows the microprocessor to distin- VPG pin is detected and used to select

unique part numbers for different thresh-

guish between resets caused by a supply one of 16 possible configurations as

old voltage combinations. Figure 1 shows

undervoltage from those due to software shown in Table 1. Recommended ±1%

a typical application of the LTC2939

malfunction. Both devices feature reset resistor values to select each configura-

monitoring 12V, 5V, 3.3V, 2.5V, 1.8V and

and watchdog timers that can be arbi- tion can also be found in Table 1.

1.2V supplies with no external resistive

trarily adjusted using external capacitors

dividers required for V1 through V4. The actual supply thresholds are set by

for greater flexibility in system design.

integrated precision dividers for 5V, 3.3V, 2.5V, 1.8V, 1.5V and 1.2V supply monitor-

Figure 1. Typical application using the LTC2939 to monitor 6 supply voltages

ing. For modes 6 (see Figure 1), 7 and 10, no external resistors are needed at

V1 the comparator inputs (V1 through V4)

to monitor the combinations of voltages

2.5V shown in Table 1. For other supply com-

V3 1.8V LTC2939 binations, uncommitted comparators (in

V4 RST

V5 WDO µPROCESSOR ADJ mode) with 0.5V thresholds allow vir-

V6 WDI tually any positive supply to be monitored VREF

as shown in Figure 3. The V4 input also

monitors negative voltages with the same

VPG GND CRT CWT

1.5% accuracy using the integrated buff-

100k 100k CRT CWT t RST = 94ms

40.2k ered reference for offset (see Figure 4). The

1% 1% 47n F 47n F t WD = 940ms

LTC2939 has two additional uncommitted

October 2010 : LT Journal of Analog Innovation | 17