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AN725

ADVANCED LOW POWER TECHNIQUES FOR SiM3L1XX DEVICES

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AN725

5.3. Low Power Mode Charge Pump

The SiM3L1xx devices feature a low-power, voltage-halving charge pump used to power most of the functions

operating in the lowest power mode (PM8). This charge pump always powers the low-frequency and 32 kHz crystal

oscillators. When in PM8, this charge pump also powers the following digital components: RTC counters and

alarms, LPTIMER0, UART0, ACCTR0, retention RAM banks, LCD0, and the Power Management Unit (PMU) and

reset controllers. When not in PM8, these modules are powered by the Memory regulator (LDO0). The analog

circuitry of the LCD and Pulse Counter is always powered directly from the VBAT pin.

The low power mode charge pump is a voltage-halving circuit which reduces the sleep-mode supply current using

two methods:

1. It provides approximately VBAT divided-by-2 to the oscillators and digital logic, reducing their inherent

power consumption.

2. It provides a transformer effect which causes the current pulled from the VBAT input to be ideally one-half

of the current actually consumed by the circuitry powered by the charge pump.

This charge pump can be modeled as a VBAT divided-by-2 supply with a series impedance of RCPLOAD.

Equation 1 shows the equation for the output voltage of the charge pump.

VCP

=

V-----B----A-----T--

2

–

ILOAD



RCPLOAD

Equation 1. Charge Pump Output Voltage

The value of RCPLOAD depends on the output drive setting field (CPLOAD) in the PMU module and the clock

selected by the RTC0 module, as shown in Table 3.

Table 3. Charge Pump Output Drive Settings

CPLOAD field

0

1

2

3

RCPLOAD Value

LFOSC (16.4 kHz) RTCOSC (32 kHz)

~600 k

~300 k

~200 k

~100 k

~90 k

~45 k

~40 k

~20 k

Overhead Current

LFOSC (16.4 kHz) RTCOSC (32 kHz)

~0 nA

~0 nA

~25 nA

~50 nA

~50 nA

~100 nA

~75 nA

~150 nA

All circuitry powered by the charge pump operates correctly at VCP voltages greater than or equal to 0.95 V. Thus,

for a given charge pump load current, Equation 2 must be true.

V-----B----A-----T--

2

–

ILOAD



RCPLOAD



0.95

V

Equation 2. Charge Pump Output Voltage Requirement

In all applications with a VBAT voltage of 2.4 V or above, the charge pump can operate at its minimum drive

impedance (CPLOAD = 3). However, as shown in Table 3, the overhead current consumed by the charge pump

increases as its drive impedance decreases. Therefore, at lower values of ILOAD, the total PM8 current

consumption can be reduced by increasing the charge pump’s drive impedance. In addition, since a higher drive

impedance corresponds to a lower output voltage for a given load current, increasing the drive impedance also

reduces the current consumed by the load circuitry.

At higher temperatures, using the low power charge pump and RTC0 can help reduce the leakage current of the

device. Reducing CPLOAD can further reduce the leakage at these temperatures.

18

Rev. 0.1

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