'I

Load Current

Increases

Normal Operation

Current Limited

'I =

L

LM3100

STOP

RUN

VIN

EN

LM3100

SNVS421F – JANUARY 2006 – REVISED DECEMBER 2009

www.ti.com

Figure 2. Shutdown Implementation

Current Limit

Current limit detection occurs during the off-time by monitoring the re-circulating current through the low-side

synchronous switch. Referring to Functional Block Diagram, when the buck switch is turned off, inductor current

flows through the load, into PGND, and through the internal low-side synchronous switch. If that current exceeds

1.9A the current limit comparator toggles, forcing a delay to the start of the next on-time period. The next cycle

starts when the re-circulating current falls back below 1.9A and the voltage at FB is below 0.8V. The inductor

current is monitored during the low-side switch on-time. As long as the overload condition persists and the

inductor current exceeds 1.9A, the high-side switch will remain inhibited. The operating frequency is lower during

an over-current due to longer than normal off-times.

in steady state. When an overload occurs, the inductor current will increase until it exceeds the current limit

threshold, 1.9A. Then the control keeps the high-side switch off until the inductor current ramps down below

1.9A. Within each on-time period, the current ramps up an amount equal to:

(6)

+

ΔI/2.

Figure 3. Inductor Current - Current Limit Operation

N-Channel Buck Switch and Driver

The LM3100 integrates an N-Channel buck (high-side) switch and associated floating high voltage gate driver.

The gate drive circuit works in conjunction with an external bootstrap capacitor and an internal high voltage

cycle to recharge the bootstrap capacitor.

10

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