Lithium-ion-battery-charging IC powered by charge-transfer, control innovations
The IC Insider: Reverse engineering the Maxim MAX8814ETA 28V linear lithium-ion battery-charger IC.
By Randy Torrance, Chipworks -- EDN, October 1, 2008
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Designers of portable electronics can't yet meet consumer demand for devices that recharge instantly, but a new breed of lithium-ion-battery-charger ICs is helping them get closer to that ideal—without jeopardizing user safety. Chipworks has recently completed an exhaustive reverse-engineering effort and analysis for one of the most advanced of these battery-charger ICs: Maxim Integrated Products' MAX8814ETA. This exclusive IC Insider article highlights some of the chip's key innovations.
The MAX8814ETA is a stand-alone, CCCV (constant-current, constant-voltage), thermally regulated linear charger, designed for charging single-cell lithium-ion batteries. It targets cellular and cordless phones, PDAs, MP3 players, digital still cameras, and USB appliances. The device integrates the current-sense circuit, MOSFET pass element, and thermal-regulation circuitry, and eliminates the reverse-blocking Schottky diode that's required with many other battery-charging ICs.
The MAX8814ETA controls the charging sequence from the prequalification state through constant-current fast-charge to 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 device achieves high flexibility by providing an adjustable fast-charge current through an external resistor. Other features include an active-low control input and an active-low input-power-source-detection output. The IC also features a booting assistant circuit that distinguishes input sources and battery connection while providing an output signal for system booting.
The MAX8814ETA accepts an input supply range from 4.25 to 28V, but disables charging if the input voltage exceeds +7V to protect against unqualified or faulty ac adapters. The IC operates over the extended temperature range of –40°C to +85°C. It is fabricated in a two-metal, single-poly BiCMOS process and mounted in a compact, 8-pin, thermally enhanced TDFN 2×2-mm package.
Chipworks has recently completed a complete circuit analysis of this chip. The IC features quite a few interesting circuits, including a temperature sensor, lots of voltage and current sensors, and a fascinating methodology for entering and sequencing through test modes. For this installment of IC Insider, however, we'll focus on two interesting circuits: the actual MOSFETs used to pass the charge from the transformer to the battery and one of the main control circuits to these MOSFETs.
Charge-transfer circuit
The charge-transfer circuit accomplishes the actual transfer of charge from the input pin to the lithium-ion battery being charged. The input pin voltage range is from 4.25 to 28V, and the output needs to eventually charge the battery to 4.2V. Hence this circuit needs to be able to handle high voltages and high currents while ensuring that the battery itself is not subjected to any voltages much higher than 4.2V. This circuit also senses the input and output current for use in the overcurrent-protection loops.
The circuit is divided into two sections: the input section and the output section, and is shown in Figure 1.
The input section consists entirely of DMOS devices, and is connected to the input supply voltage pin IN_PIN. DMOS are field effect transistors that are capable of handling large voltages. They use much longer channel lengths than standard CMOS transistors, and have very large dimensions from source to drain to keep the electric fields low. Because the input voltage can be as high as 28V, all transistors in this input section must be DMOS.
Referring to the schematic in Figure 1, DMOS X1402 and X1395 carry the current from the input pin to the output section. X1369 has been disconnected via a metal option. The input current sensor (components X1367, X1368) is a part of the input-overcurrent-protection loop. The sense current IN_PIN_SEN is fed to the overcurrent-detection circuit, which can then turn off the input DMOS devices. Interestingly enough, the total W/L (width to length) ratio of the sense DMOS transistors is 2000× smaller than the total W/L ratio of the charge-transfer DMOS. These devices can be seen highlighted in white in a planar die image, taken at the polysilicon layer (Figure 2).
The charge transfer DMOS are the three large structures on the left. The current sense DMOS are the two small structures on the bottom left, and one of these is shown at higher magnification in Figure 3.
The output section of the schematic consists of lower-voltage NMOS transistors, because the DMOS devices bring the voltage down to a reasonable level. This output section supplies the charging current to the battery through the pin BATT_PIN. NMOS N839 and N862 carry the current to the battery pin, with N846 being disconnected via a metal option. The output current sensor (components N865, N876, N847, and N854) is a part of the charge-current-regulation loop. The sense current BATT_SEN is fed to the charge-current-regulation circuit which adjusts the charging-control circuits. The total W/L ratio of these sensor transistors is 112× smaller than the total W/L ratio of the charging transistors. These NMOS transistors are the devices highlighted in white on the right side of Figure 2.
Charging-control circuit
The charging-control circuit selects either the prequalification charge (precharge) mode or fast-charge mode, based on the battery voltage first threshold detect signal. When this signal is low, the circuit selects the precharge mode. This mode enables a precharge driver and its charge pump, and a control voltage directly controls the pass MOSFETs, based on a reference current. When the battery voltage reaches its threshold, the control circuit switches to the fast-charge mode. This disables the precharge section and enables a soft-start clock generator. This circuit first generates a few CLKFCH clock periods of frequency FOSC/2, and then switches the frequency to FOSC/8.
After 256 periods of the oscillator clock, the output signal of the soft-start timer goes high and switches the CLKFCH frequency to FOSC/4. The clock CLKFCH is fed to the fast-charge driver-control circuit, which splits it into four clocks directly controlling the fast-charge pump. The generated voltage VCTICH (controlling the pass MOSFETs) is driven higher and the charge current ramps up to the full charging current.
Showing all this circuitry really requires 10 schematics; Figure 4 shows one of the interesting circuits, the fast-charge driver, which drives the VCTICH charge-passing MOSFET control signal during fast-charge mode.
This signal is regulated to allow the fastest possible charging of the battery, within the many constraints of power, temperature, and so on. Because this control signal can go quite high to allow the fastest charging, all the high-voltage nodes require DMOS transistors. This creates an interesting layout, where DMOS and CMOS transistors mix together, as shown in Figure 5. The elements highlighted in white are those actually used in the fast-charge-driver circuit.
The published data on the Maxim MAX8814 28V Linear Li+ Battery Charger shows some of the most impressive specifications on the market. Chipworks' complete analysis of this device presents some interesting innovation across 67 organized schematics derived from 4429 annotated devices (transistors, resistors, capacitors). We look forward to bringing you more of the latest innovative circuits in EDN's ongoing IC Insider series.
