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Power Save Mode
Low Battery Detector Circuit—LBI/LBO
Low-EMI Switch
LDO
LDO Enable
SLVS431B – JUNE 2002 – REVISED JANUARY 2008
The SKIPEN pin can be used to select different operation modes. To enable the power save mode, SKIPEN
must be set high. Power save mode is used to improve efficiency at light loads. In power save mode, the
converter only operates when the output voltage trips below a set threshold voltage. It ramps up the output
voltage with several pulses, and goes again into power save mode once the output voltage exceeds the set
threshold voltage. The power save mode can be disabled by setting the SKIPEN to GND.
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.
The device integrates a circuit that removes the ringing that typically appears on the SW node when the
converter enters discontinuous current mode. In this case, the current through the inductor ramps to zero and the
rectifying PMOS switch is turned off to prevent a reverse current flowing from the output capacitors back to the
battery. Due to the remaining energy that is stored in parasitic components of the semiconductor and the
inductor, a ringing on the SW pin is induced. The integrated antiringing switch clamps this voltage to VBAT and
therefore dampens ringing.
The built-in LDO can be used to generate a second output voltage derived from the dc/dc converter output, from
the battery, or from another power source like an ac adapter or a USB power rail. The LDO is capable of being
back biased. This allows the user just to connect the outputs of dc/dc converter and LDO. So the device is able
to supply the load via dc/dc converter when the energy comes from the battery and efficiency is most important
and from another external power source via the LDO when lower efficiency is not critical. The LDO must be
disabled if the LDOIN voltage drops below LDOOUT to block reverse current flowing. The status of the dc/dc
stage (enabled or disabled) does not matter.
The LDO can be separately enabled and disabled by using the LDOEN pin in the same way as the EN pin at the
dc/dc converter stage described above. This is completely independent of the status of the EN pin. The voltage
levels of the logic signals which need to be applied at LDOEN are related to LDOIN.
Copyright 2002–2008, Texas Instruments Incorporated
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