Sub-1V 555-like timer includes LED driver and charger

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Semtech has introduced an analogue timer chip that runs from 0.9V, but can drive LEDs at over 3V.

Called SX8122, to the timer it adds: a simple battery charger, a boost converter, and a power-down controller.

“It is designed for on/off button debouncing, delayed start-up timing, programmable sequencing and timing-out in toys, remote control, electric toothbrushes, buzzers, and LED pointers,” said Semtech.

The timer part is very similar to a 555, and there is a guidance document to describe the differences.

“The 555 timer is one of the most popular ICs ever, because it is easy to use, cost-effective and a proven performer,” said Semtech engineering manager Michel Chevroulet. “By embedding unique low-voltage features, the SX8122 extends the versatility of the 555 timer in small light battery-enabled consumer products.”

He also said: “The SX8122 is the industry’s first analogue timer embedding low-voltage features for single AA/AAA cell-powered appliances.”

This last statement is not actually true, as a few years ago Zetex – now part of Diodes – employed original 555 designer Hans Camenzind to develop an 0.9V version from scratch.

This was introduced as the ZSCT1555.

What is unique about Semtech’s offering is the LED driver which, whenever the main (motor-driving) output is high, puts out a square wave. itemid-54174-getasset.jpg

The intention is that this square wave be used to drive an external inductor via a transistor, forming the simple boost converter that drives LEDs or other loads whose forward voltage is greater than the supply.

The comparator/charger is also unusual.

Described by the firm as “a battery overload indicator for slow NiMH or NiCd charger management”, It is set to switch at 1.41V, with hysteresis of 30mV to dissuade oscillation.

The comparators output is also an input as 4mA bidirectional switches connect it either to the positive rail or the 0V rail.

In its simplest for, see diagram below, the comparator acts as a trickle-charger, connecting the charging source through the external resistor to battery via the internal chip positive rail.
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For safety, should the battery voltage get above 1.4V the comparator switches over and its lower output transistor (not shown) shorts-out the charger to 0V via its resistor.

An application note adds two transistors and two resistors to increase charge current while decreasing the shorting current.

Hidden from the simplified circuit shown are power monitoring blocks that control chip behaviour at power-up and down.

“It starts controlling I/Os at 0.6V to eliminate unexpected behaviour in the external components and prevent current drain during the start-up phase,” said Semtech. “At power down from a low battery, the I/Os are controlled again down to 0.6 V and the current consumption is guaranteed less than 10µA over the full industrial temperature range to avoid battery leakage.”

Operating voltage range is 0.9-2.0V, although the chip is only fully specified from 1.0 to 1.8V.

Active current is 50-80µA (Zetex’ 555 runs on 74µA), operating temperature range is -40 to 85°C, and the package is an 8pin SOIC.

It joins two fixed duration 0.9V timer chips from Semtech, the SX8120 and SX8121.

Tags: charger, led driver, Semtech

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