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LM3017

SNOSC66D – MARCH 2012 – REVISED SEPTEMBER 2016

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7.3.6 Current Limit Protection

The LM3017 implements current limit protection by controlling the pass FET, Q2.

In boost mode the LM3017 features both cycle-by-cycle current limit and short-circuit protection. Unlike most

boost regulators, the LM3017 can protect itself from short circuits on the output by shutting off the pass FET. The

boost current limit, defined by VCL= 170 mV in Electrical Characteristics, turns off the boost FET for normal

overloads on a cycle-by-cycle basis. The current is limited to VCL / RSEN until the overload is removed. If the

output must be shorted, or otherwise pulled below VIN, the inductor current has a tendency to run-away. This is

prevented by the short-circuit protection feature, defined as VSC = 200 mV in Electrical Characteristics. When this

current limit is tripped, the current is limited to VSC / RSEN by controlling the pass FET. If the short persists for

TSC > 450 µs the pass FET is latched off. In this way, the current is limited to VSC / Rsen until the short is removed

or the time of TSC = 450 µs is completed. Pulling the EN/MODE pin low (< 0.4 V, typical) is required to reset this

short-circuit latch-off mode. The delay of TSC = 450 µs helps to prevent nuisance latch-off during a momentary

short on the output.

7.4 Device Functional Modes

7.4.1 Shutdown Mode

Pulling the EN/MODE pin to less than 0.4 V (typical), during any mode of operation, places the part in full

shutdown mode. The boost regulator and the pass FET is off and the load is disconnected from the input supply.

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

7.4.2 Boost Mode

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

This is the run mode for the boost regulator. Note that the LM3017 always starts in pass-through and transitions

to boost mode.

7.4.3 Standby Mode

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

regulator is off and the pass MOSFET is 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.

7.4.4 Start-Up Boost Mode

During start-up in boost mode, peak inductor current may be higher compared to normal operation. To allow for

this, current limit levels and timing are different during start-up. The current limit is defined by VLIM2 = 100 mV

(typical) in Electrical Characteristics, for the first TLIM2 = 3.6 ms (typical). The current is limited to VLIM2 / RSEN, for

this period. Once the TLIM2 = 3.6 ms (typical) timer has finished, the current limit is increased to VSC = 200 mV

(typical). For the first TLIM2 = 3.6 ms (typical) of the start-up, the latch-off feature is not enabled; however, the

current is always limited to VLIM2 / RSEN. This allows the part to start up normally. If the current limit is still tripped

at the end of TLIM2 = 3.6 ms (typical) , the TSC = 900 µs (typical) timer is started. Once the TSC = 900 µs (typical)

time has expired, the pass FET (Q2) is latched off. This gives a total current-limited time of TSC + TLIM2 = 4.05 ms

(typical), in cases where the LM3017 is started into a short circuit at the output.

7.4.5 Pass-Through Mode

In pass-through mode the power path is protected from shorts and overloads by the current limit defined as

VLIM1 = 85 mV (typical) in Electrical Characteristics. When this current limit is tripped, the current is limited to

VLIM1 / RSEN by controlling the pass FET. If the short persists for TLIM1 > 900 µs (typical) the pass FET (Q2) is

latched off. In this way, the current is limited to VLIM1 / RSEN until the short is removed or the time of

TLIM1 = 900 µs (typical) is completed. Pulling the EN/MODE pin low (0.4 V, typical) is required to reset this latch-

off mode.