Datasheet

I
CL
I
PK
'I
I
OCL
I
OUT
Inductor Current
Load Current
Increases
Normal Operation
Current Limited
'I =
(V
IN
- V
OUT
) x t
ON
L
LM3100
STOP
RUN
V
IN
VIN
EN
LM3100
SNVS421G –JANUARY 2006–REVISED APRIL 2013 www.ti.com
Figure 20. 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.
Figure 21 illustrates an inductor current waveform, the average inductor current is equal to the output current,
I
OUT
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)
During this time the LM3100 is in a constant current mode, with an average load current (I
OCL
) equal to 1.9A
+ΔI/2.
Figure 21. 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
diode. A 33 nF capacitor (C
BST
) connected between BST and SW pins provides voltage to the high-side driver
during the buck switch on-time. During each off-time, the SW pin falls to approximately -1V and C
BST
charges
from the V
CC
supply through the internal diode. The minimum off-time of 260ns ensures adequate time each
cycle to recharge the bootstrap capacitor.
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