ion (Li+) battery. The MAX8814 integrates the current-sense circuit, MOSFET pass

element, thermal-regulation circuitry, and eliminates the reverse-blocking Schottky diode to

create the simplest and smallest charging solution for handheld equipment.

The MAX8814 controls the charging sequence from the prequalification state through

constant-current fast-charge and the final constant-voltage charge. Proprietary thermal-

regulation circuitry limits the die temperature during fast-charging or when the IC is

exposed to high ambient temperatures, allowing maximum charging current without

damaging the IC.

The MAX8814 achieves high flexibility by providing an adjustable fast-charge current

through an external resistor. The fast-charge current is set to 32mA. For a fully discharged

battery, the GSR device can be fully charged in less than two hours.

A fully charged battery can last about 15 hours with the default firmware settings. In the

default settings, the device measures the skin impedance and temperature every 1s, and

transmits the data to a mobile device through BLE wireless interface.

When the device is being charged, the yellow LED D3 turns on and the charging indication

icon on the mobile app is also turned on.

AC Impedance Measurement Circuit and Settings

The AC impedance measurement block diagram is shown in Figure 2.

Referring to the schematic, the 12-bit DAC0 generates the excitation sinusoidal signal with

a 1V peak-to-peak magnitude. The microcontroller DMA engine makes the direct digital

synthesis possible. The DAC0 output signal is buffered by OP AMP D. R17, R18, C25,

C26, and OP AMP A construct a second-order lowpass filter (LPF). C27 blocks the DC

component of the excitation signal. Four internal SPST switches dynamically reconfigure

the load to either the calibration path or human skin load. The 8-bit DAC2 generates the

common-mode bias before ADC input. C29 and R25 construct a first-order LPF and gain

control. Finally, R41, R42, and C47 work as a LPF before ADC input.

The MAXREFDES73# sets the default excitation frequency at 7.8125kHz. The hardware

settings for this single fixed-frequency measurement are as follows:

• Both ADC and DAC voltage references = 2.048V

• DAC0 power level = 2, sine wave output frequency = 7.8125kHz, voltage = 1V peak-

to-peak

• DAC2 DC output voltage = 1.12V

• ADC sample rate = 31.25kHz, decimation = 4, PGA gain = 2

• Clock source: 24MHz internal relaxation oscillator