Datasheet

0.62A
Load Current
Increases
I
PK
I
OCL
I
O
'I
Normal Operation
Current Limited
Inductor Current
STOP
RUN
RON/SD
Input
Voltage
LM2694
VIN
R
ON
LM2694
SNVS444A –MAY 2006–REVISED APRIL 2013 www.ti.com
Figure 11. Shutdown Implementation
Current Limit
Current limit detection occurs during the off-time by monitoring the recirculating current through the free-wheeling
diode (D1). Referring to Typical Application Circuit and Block Diagram, when the buck switch is turned off the
inductor current flows through the load, into SGND, through the sense resistor, out of ISEN and through D1. If
that current exceeds 0.62A the current limit comparator output switches to delay the start of the next on-time
period if the voltage at FB is below 2.5V. The next on-time starts when the current out of ISEN is below 0.62A
and the voltage at FB is below 2.5V. If the overload condition persists causing the inductor current to exceed
0.62A during each on-time, that is detected at the beginning of each off-time. The operating frequency is lower
due to longer-than-normal off-times.
Figure 12 illustrates the inductor current waveform. During normal operation the load current is Io, the average of
the ripple waveform. When the load resistance decreases the current ratchets up until the lower peak reaches
0.62A. During the Current Limited portion of Figure 12, the current ramps down to 0.62A during each off-time,
initiating the next on-time (assuming the voltage at FB is <2.5V). During each on-time the current ramps up an
amount equal to:
ΔI = (V
IN
- V
OUT
) x t
ON
/ L1 (8)
During this time the LM2694 is in a constant current mode, with an average load current (I
OCL
) equal to 0.62A +
ΔI/2.
Figure 12. Inductor Current - Current Limit Operation
N - Channel Buck Switch and Driver
The LM2694 integrates an N-Channel buck switch and associated floating high voltage gate driver. The peak
current allowed through the buck switch is 1.5A, and the maximum allowed average current is 1A. The gate
driver circuit works in conjunction with an external bootstrap capacitor and an internal high voltage diode. A 0.022
µF capacitor (C4) connected between BST and SW provides the voltage to the driver during the on-time. During
each off-time, the SW pin is at approximately -1V, and C4 charges from V
CC
through the internal diode. The
minimum off-time of 265 ns ensures a minimum time each cycle to recharge the bootstrap capacitor.
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