The author of this article
Victor Khasiev
Senior application engineer for power products
Linear Technology Corporation (now part of Analog Devices)
introduction
    In terms of fully functional lithium-ion battery chargers, the main obstacle encountered by some designers is the lack of automatic or autonomous triggering function. For example, the controller does provide C/10 current detection, but does not interrupt the charging process when the charging current drops to a C/10 value. It also lacks the automatic restart required for a fully qualified charger.
    The LTC4012 is a very popular chip used to charge lithium-ion batteries in a variety of applications. This controller provides grid drive and current detection inputs to establish a power link for a step-down topology. The LTC4012 also provides control signals, such as the state of charge and the presence of the adapter. The device also offers a range of useful functions, which are detailed in its data sheet.
    In applications that do not use the CPU, designers either simply refuse to use the LTC4012 or develop their own solutions, which in some cases is not enough. For example, some designers may use a resistor on the PROG pin to detect the charging current, but connecting the PROG pin to a low-cost, low-impedance buffer will fundamentally distort the measurement.
    The purpose of this article is to illustrate a simple automatic restart solution that does not use PROG pins and uses logic signals for C/10 current detection. This detection is based on the "digital C/10 indicator" introduced in the LTC4012 data sheet, as shown in the block diagram.

Automatic restart circuit block diagram
    The block diagram of the aforementioned circuit is shown in Figure 1. The circuit is based on a D flip-flop. As the charge current drops to the pre-set C/10, the D flip-flop clock changes to high, and the external transistor Q1 pulls the RUN pin to ground and turns off the LTC4012. The battery is charged. The voltage is close to maximum. As the charger is turned off, the battery begins to discharge, and eventually the minimum voltage is reached, the D trigger is cleared to zero, and then the Q1 is turned off, and the charging cycle is repeated.