LP8725

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SNVS618G – DECEMBER 2009 – REVISED MAY 2013

Internal Synchronous Rectification

While in PWM mode, the DC-DC convertor uses an internal NFET as a synchronous rectifier to reduce rectifier

forward voltage drop and associated power loss. Synchronous rectification provides a significant improvement in

efficiency whenever the output voltage is relatively low compared to the voltage drop across an ordinary rectifier

diode.

Current Limiting

A current limit feature allows the DC-DC convertor to protect itself and external components during overload

conditions. PWM mode implements current limit using an internal comparator that trips at 1.05A (typ.) assuming

enters a timed current limit mode where the switching frequency will be one fourth, and NFET synchronous

rectifier is disabled, thereby preventing excess current and thermal runaway.

The current limit for each DC-DC convertor is selectable via serial interface by registers 0x09 bits 6, 7 and 0x0B

bits 6, 7. The current limit selected should be ~ 1.5x to2x greater than the output current required.

ECO Mode Operation

By default the DC-DC converter will be in Auto (ECO/PWM) Mode . By doing so the part switches from ECO

(ECOnomy) state to PWM (Pulse Width Modulation) state based on output load current. At light loads (less than

75mA approx) the converter enters ECO mode. In this mode the part operates with low Iq. During ECO operation

the converter positions the output voltage slightly higher (+30mV typ.) than the nominal output voltage in PWM

operation. Because the reference is set higher, the output voltage increases to reach the target voltage when the

part goes from sleep state to switching state. Once this voltage is reached the converter enters sleep mode,

thereby reducing switching losses and improving light load efficiency. The output voltage ripple is slightly higher

in ECO mode (30mV p–p typ.).

Figure 14. Typical ECO Operation

Note that ground noise may impact quiescent current in ECO mode and care at board layout is important to

minimize this risk. See section on layout guidelines.

Startup

The LP8725 bucks have a ‘soft-start’ feature to limit the in-rush current. This prevents large current spikes and

voltage overshoot and also limits the cases in which the inductor may saturate. At or close to 0V the inrush

current to the capacitor (and load) is limited to its short circuit protection limit of typically 500 mA. Above a

threshold of around 150 mV, the current limit is increased to the buck’s peak current limit set by the control

registers. (For a 600 mA max load this is set to approx. 1050 mA.)

While in short-circuit protection, the output switches off, but will continue to attempt to restart. If the total

capacitance on the buck output is large or if the inductor value drops too low, the threshold might never be

reached, so the buck may not start. See Inductor Selection and Output Capacitor Selection sections.

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