LM3017

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SNOSC66C – MARCH 2012 – REVISED MARCH 2013

TRUE SHUTDOWN

The LM3017 incorporates circuitry to control a high side NMOS transistor in series with the inductor. This feature

is used to dis-connect the load from the input supply and protect the system from shorts on the output. Using an

NMOS, rather than a PMOS transistor, saves the use of a diode from the inductor to ground. When the NMOS is

turned-off, the inductor brings the source belowground, keeping it on until the current is safely brought to zero. A

OPERATION OF THE EN/MODE PIN

The EN/MODE pin is used to control the modes of the regulator by driving the high side gate (VG pin) to enable

or disable the output through the pass MOSFET. Furthermore it defines the current limit for each operation mode

(see next section). The following table shows the modes versus the voltage on the EN/MODE pin:

EN/MODE PIN VOLTAGE

MODE

≤ 0.4V

Shutdown

1.6 V to 2.2 V

Boost

≥2.6 V

Pass-through

Figure 13 shows the output voltage behavior in the various operation modes.

SHUTDOWN MODE

Pulling the EN/MODE pin to less than 0.4V (typ.), during any mode of operation, will place the part in full

shutdown mode. The boost regulator and the pass FET will be off and the load will be disconnected from the

input supply. In this mode, the regulator will draw a maximum of 1µA from the input supply.

BOOST MODE

The boost regulator can be turned on by bringing the EN/MODE pin to greater than 1.6V, but less than 2.2V.

This is the run mode for the boost regulator. Note that the LM3017 will always start in pass-through and transition

to boost mode.

STANDBY MODE

Setting the EN/MODE pin to greater than 2.6V (typ.), will place the part in pass-through mode. The boost

regulator will be off and the pass MOSFET will be on. During this mode, the load is connected to the input supply

through the inductor and power diode, and is fully protected from output short circuits.

EN/MODE CONTROL

As stated previously, the EN/MODE pin controls the state of the LM3017. As with any digital input , the voltage

on this pin must not be allowed to slowly cross the various thresholds. Although hysteresis is used on this input,

slowly varying signal may cause unpredictable behavior. Also, the EN/MODE pin must not be allowed to float.

One way to control the LM3017, from digital logic, is to use the circuit shown in Figure 14. The resistor values

are adjusted based on the above table and the logic supply used. The MOSFET can be any small signal device,

such as the 2N7002.

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