LM3691

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SNVS506H – MAY 2008 – REVISED SEPTEMBER 2011

Internal Synchronous Rectification

While in PWM mode, the LM3691 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 LM3691 to protect itself and external components during overload conditions.

PWM mode implements current limit using an internal comparator that trips at 1500 mA (typ). If the output is

shorted to ground and output voltage becomes lower than 0.3V (typ.), the device 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.

ECO OPERATION

Setting mode pin low places the LM3691 in Auto mode. By doing so the part switches from ECO (ECOnomy)

state to FPWM (Forced Pulse Width Modulation) state based on output load current. At light loads (less than 50

mA), 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 (+30 mV typ.) than the nominal output voltage in FPWM

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 (30 mV peak–peak ripple typ.).

Figure 46. Typical ECO Operation

FORCED PWM MODE

Setting Mode pin high (>1.2V) places the LM3691 in Forced PWM. The part is in forced PWM regardless of the

load.

SHUTDOWN MODE

Setting the EN input pin low (<0.4V) places the LM3691 in shutdown mode. During shutdown the PFET switch,

NFET switch, reference, control and bias circuitry of the LM3691 are turned off. Setting EN high (>1.2V) enables

normal operation. When turning on the device with EN soft-start is activated. EN pin should be set low to turn off

the LM3691 during system power up and under-voltage conditions when the supply is less than 2.3V. Do not

leave the EN pin floating.

SOFT-START

The LM3691 has a soft-start circuit that limits in-rush current during startup. Output voltage increase rate is 30

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