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SLVSA32C – OCTOBER 2009 – REVISED MAY 2010
Figure 20. Low Power Mode
Figure 21. Low Power Mode (Zoom In)
Any transition from low-power mode to active mode CCM occurs within 5 s (typical). In low-power mode, the
converter operates as a hysteretic controller with the threshold limits set by VReg_UV (see
Equation 10, Figure 4and Figure 22), for the lower limit and ~VReg for the upper limit. To ensure tight regulation in the low-power mode, R2 and R3 values are set accordingly (see discussion on Noise Filter on RST_TH and OV_TH Terminals). The
device operates in both automatic (LPM pin is connected to ground) and digitally controlled (status of LPM pin is
controlled by an external device, for example by a microcontroller) low-power mode. The digital low-power mode
can over-ride the automatic low-power mode function by applying the appropriate signal on the LPM terminal.
The part goes into active mode CCM for at least 100 s, whenever RST_TH or VReg_UV is tripped.
Table 2. LPM Pin Status
LPM PIN STATUS
MODES OF OPERATION
Device is forced in normal mode.
At light loads, the device operates in DCM with a switching frequency determined by the external
High
resistor connected to RT pin.
At very light loads, the device operates in PSM with a reduced switching frequency (see
Figure 14).Device automatically changes between normal mode and low-power mode depending on the load
Low or open
current.
Table 3. Modes of Operation
MODES OF OPERATION
DESCRIPTION
All circuits including overvoltage threshold circuit (OV_TH) are enabled.
At heavy loads, the device operates in continuous conduction mode irrespective of the status of LPM
pin.
Normal mode (active mode)
OR
At light loads, the device operates in discontinuous conduction mode (DCM) only if LPM pin is externally
set high.
OV_TH circuit is disabled.
Low-power mode
The device is in DCM, and LPM pin should be forced low.
When the device is operating in low-power mode, and if the output is shorted to ground, a reset is asserted. The
thermal shutdown and current limiting circuitry is activated to protect the device. The LPM pin is active low and is
internally pulled down; therefore, the low-power mode is automatically enabled unless this pin is driven high
externally (for example, by a microcontroller) and the device is in continuous conduction mode. However, the
low-power mode operation is initiated only when the device enters discontinuous mode of operation at light
loads, and the LPM pin is low (or connected to ground).
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