0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

VBAT = 5 V

VBAT = 3.6 V

VBAT = 2.4 V

VBAT = 1.8 V

VBAT = 1.2 V

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SLVS451G – SEPTEMBER 2003 – REVISED DECEMBER 2014

Feature Description (continued)

10.3.5 Undervoltage Lockout

An undervoltage lockout function prevents device start-up if the supply voltage on VBAT is lower than

approximately 0.8 V. When in operation and the battery is being discharged, the device automatically enters the

shutdown mode if the voltage on VBAT drops below approximately 0.8 V. This undervoltage lockout function is

implemented in order to prevent the malfunctioning of the converter.

10.3.6 Softstart and Short Circuit Protection

When the device enables, the internal startup cycle starts with the first step, the precharge phase. During

precharge, the rectifying switch is turned on until the output capacitor is charged to a value close to the input

voltage. The rectifying switch is current limited during that phase. The current limit increases with the output

voltage. This circuit also limits the output current under short circuit conditions at the output. Figure 12 shows the

typical precharge current vs output voltage for specific input voltages:

Figure 12. Precharge and Short Circuit Current

After charging the output capacitor to the input voltage, the device starts switching. If the input voltage is below

1.4 V the device works with a fixed duty cycle of 50% until the output voltage reaches 1.4 V. After that the duty

cycle is set depending on the input output voltage ratio. Until the output voltage reaches its nominal value, the

boost switch current limit is set to 40% of its nominal value to avoid high peak currents at the battery during

startup. As soon as the output voltage is reached, the regulator takes control and the switch current limit is set

back to 100%.

10.3.7 Low Battery Detector Circuit—LBI/LBO

The low-battery detector circuit is typically used to supervise the battery voltage and to generate an error flag

when the battery voltage drops below a user-set threshold voltage. The function is active only when the device is

enabled. When the device is disabled, the LBO pin is high-impedance. The switching threshold is 500 mV at LBI.

During normal operation, LBO stays at high impedance when the voltage, applied at LBI, is above the threshold.

It is active low when the voltage at LBI goes below 500 mV.

The battery voltage, at which the detection circuit switches, can be programmed with a resistive divider

connected to the LBI pin. The resistive divider scales down the battery voltage to a voltage level of 500 mV,

which is then compared to the LBI threshold voltage. The LBI pin has a built-in hysteresis of 10 mV. See the

application section for more details about the programming of the LBI threshold. If the low-battery detection

circuit is not used, the LBI pin should be connected to GND (or to VBAT) and the LBO pin can be left

unconnected. Do not let the LBI pin float.

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