Low-power device power backup solution based on MAX16024

1 Introduction
In low-power devices, power supply stability is one of the key factors affecting device performance. An efficient power backup system can provide low-power devices with stable power management performance and flexible power supply configuration options. Therefore, a low-power device power backup solution based on MAX16024 is proposed here.

2 MAX16024 Introduction
MAX16024 is a low-power power backup device that can provide regulated output and up to 100 mA output current. MAX16024 contains a low-dropout regulator, microprocessor (μP) reset circuit and power switching circuit, as well as other functions such as manual reset, power failure detection comparator and battery power indicator. This device can also reduce the number of external components, reduce circuit board space and improve reliability.
The input voltage of MAX16024 is 1.53~5.5 V, and it can provide fixed 1.2 V, 1.8 V, 2.5 V, 3.0 V and 3.3 V standard output voltage. MAX16024 can set the output voltage through an external resistor divider, and all outputs are in push-pull or open-drain configuration. MAX16024 operates in an industrial temperature range of 40℃ to +85℃ and is available in 8-pin or 10-pin TDFN packages. Figure 1 shows the pin configuration and internal structure of MAX16024. The following is a description of the pin functions of the MAX16024:
CEIN (Pin 1): Chip-select input. CEIN selects the circuit input; when not in use, connect it to GND or OUT.
Vcc (Pin 2): Supply voltage input. Use a 0.1μF capacitor to bypass Vcc to GND.
BATT (Pin 3): Backup power input. If Vcc is below the reset threshold (VTH), VBATT ≥ Vcc, and the regulator enters the unregulated region, the regulator is powered by BATT; if VBATI < Vcc, the regulator is powered by Vcc. Use a 0.1μF capacitor to bypass BATT to GND.
MR (Pin 4): Active low, manual reset input. When MR is pulled low, RESET is triggered to a low level; after MR changes from low to high, RESET remains low during the reset timeout period. When not in use, connect MR to OUT or leave it floating. MR is internally connected to Vcc through a 30 kΩ pull-up resistor.
SET (Pin 5): Input terminal used to set the output voltage. For the fixed-output voltage versions (MAX16024_TB_), SET is connected to GND when not in use. For the MAX16024_TB_, SET is connected to an external resistor-divider that can set the output voltage between 1.8 and 5.25 V.
BATT ON (Pin 6): Active-high battery-indicator output. BATT ON goes high when the device enters power-backup mode.
GND (Pin 7): Ground.
RESET (Pin 8): Active-low reset output. RESET asserts low when Vcc is below the reset threshold or MR is pulled low. RESET remains low during the reset timeout period after Vcc is above the reset threshold and MR is pulled high. RESET is available in push-pull and open-drain output options.
OUT (Pin 9): Linear regulator output voltage. All devices are available in factory-preset fixed output voltages of 1.2 V, 1.8 V, 2.5 V, 3.0 V, and 3.3 V.

CEOUT (Pin 10): Active Low Chip Select Output. CEOUT is low only when CEIN is low and a reset state is not entered. When a reset is asserted, if CEIN is low, CEOUT will remain low for 12 μs (typical) and then go high, or will transition high when CEIN goes high, whichever occurs first.

In the typical connection of MAX16024, OUT is used to power SRAM. If VCC is higher than the reset threshold (VTH), or VCC is lower than VTH but higher than VRATT, the regulator will be powered by VCC. If VCC < VRATT, the regulator enters the unregulated state, and the regulator will be powered by BATT. When OUT is powered by VCC, it can output a current of up to 100mA.
(1) Backup power switching In the event of power failure or power failure, it is often necessary to save the data content in RAM. BATT of MAX16024 has a backup power switching circuit, which will automatically switch to the backup power supply when VCC drops. MAX16024 has a BATT ON output, which goes high when entering the power backup mode. The device will switch to the power backup mode when one of the following conditions is met: 1) VCC is lower than the reset threshold; 2) VCC is lower than VRATT; 3) The regulator enters the unregulated state (except for the 1.2 V output voltage version).
(2) Chip select signal gating The MAX16024 provides gating control of the internal CE signal to prevent erroneous data from being written to the CMOS RAM when the power fails or is disconnected. During normal operation, the CE gating is enabled and transmits CE transients directly to the output. When the reset output is triggered, the gating channel is disabled to avoid destroying the data in the CMOSRAM, and CEOUT is pulled high to OUT through an internal current source. The 1.5 ns transmission delay between CEIN and CEOUT allows the device to work with most microprocessors and high-speed DSPs. In normal operating mode (no reset), CEIN is connected to CEOUT through a low on-resistance transmission gate. During reset, if CEIN is high, CEOUT will remain high in the reset state regardless of subsequent changes. During reset, if CEIN is low, CEOUT will remain low for 12μs to complete the read/write operation. After the 12μs delay, CEOUT will go high, and the output will remain high during reset regardless of subsequent changes in CEIN. When CEOUT is disconnected from CEIN, CEOUT is pulled up to OUT. The transmission delay of the chip select circuit depends on the source impedance driving CEIN and the capacitive load of CEOUT. Reducing the capacitive load of CEOUT will minimize the transmission delay, and a driver with a low impedance output should be used.
(3) Manual reset input RESET is triggered when it is logic low in the MAX16024. When MR is logic low, RESET remains in the reset state. After MR goes high, RESET is released after a minimum timeout period of 145 ms (tRP). MR is internally connected to VCC through a 30 kΩ pull-up resistor. MR can be driven by TTL/CMOS logic levels or open-drain/collector outputs. A normally open momentary switch can be connected between MR and GND to provide a manual reset function without the need for external debounce circuitry. If driven by long cables or the device is operating in a noisy environment, a 0.1μF capacitor needs to be connected between MR and GND to provide additional noise suppression.
(4) Battery connection indicator When entering power backup mode, the MAX160224's BATT ON output goes high to indicate the power switching status.
(5) Battery preservation Before the first connection to VCC, the MAX16024's battery preservation function ensures that the backup battery is not connected to the internal circuitry and OUT, ensuring that the backup battery connected to BATT is brand new when the final product is used for the first time. The internal preservation latch function maintains a latched state to prevent BATT from powering OUT before VCC is powered on for the first time. When VCC is subsequently turned off, BATT begins to power OUT.
(6) Reset output The MAX16024's P monitoring circuit will trigger a processor reset during power-on, power-off, and low-voltage conditions to avoid code execution errors. When VCC is below the reset threshold, RESET is triggered and continues to remain low for at least 145 ms (tRP) after VCC rises above the reset threshold. RESET is also triggered when MR is low.

(7) Regulator output The MAX16024 device provides fixed output voltages of 1.2 V, 1.8 V, 25 V, 3.0 V, and 3.3 V, and can provide a maximum load current of 100 mA. At the same time, the MAX16024 provides fixed and adjustable output voltage versions. The output voltage is set using an external resistor divider network connected between OUT, SET, and GND, and can be adjusted between 1.8 and 525 V. Connecting SET to GND selects the fixed output voltage version.

3 Application Examples
Figure 2 shows a power backup circuit based on MAX16024.

In the power backup circuit of Figure 2, the MAX16024 can provide short-circuit protection with a typical value of 10 s. If OUT is shorted to ground for more than 10 s, the device may be damaged. Use 0.1μF capacitors to bypass VCC and BATT to GND respectively. Connect a 10μF low-ESR capacitor between OUT and GND, and place all capacitors as close to the device as possible. When VCC is higher than VTH, removing the backup power supply will not trigger a reset. When VCC remains above the reset threshold, the circuit will not enter the power backup mode. When a short negative VCC voltage appears, the power backup circuit can suppress short-term disturbances and provide additional transient protection capabilities.
In the power supply stability test of the circuit, the circuit power supply is stable, and there is a good linear relationship between the input voltage and the output voltage (Figure 3); during the chip select reset process, the circuit output voltage transition is stable, as shown in Figure 4.

4 Conclusion
The MAX16024 is ideal for providing backup power to memory devices that store sensitive data, such as static random access memory (SRAM) or real-time clock (RTC). When the power supply voltage drops, the regulated output powered by VCC switches to the backup power supply.