Datasheet

0.64A
Load Current
Increases
I
PK
I
OCL
I
O
'I
Normal Operation
Current Limited
Inductor Current
STOP
RUN
RON/SD
Input
Voltage
LM34919
VIN
R
ON
LM34919
SNOSAY2E –MAY 2007–REVISED FEBRUARY 2013 www.ti.com
Figure 13. 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 the 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 0.64A the
current limit comparator output switches to delay the start of the next on-time period. The next on-time starts
when the current out of ISEN is below 0.64A and the voltage at FB is below 2.5V. If the overload condition
persists causing the inductor current to exceed 0.64A 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 14 shows 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
0.64A. During the Current Limited portion of Figure 14, the current ramps down to 0.64A 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 (7)
During this time the LM34919 is in a constant current mode, with an average load current (I
OCL
) equal to 0.64A +
ΔI/2.
Generally, in applications where the switching frequency is higher than ≊300 kHz and uses a small value
inductor, the higher dl/dt of the inductor's ripple current results in an effectively lower valley current limit threshold
due to the response time of the current limit detection circuit. However, since the small value inductor results in a
relatively high ripple current amplitude (ΔI in Figure 14), the load current (I
OCL
) at current limit is typically in
excess of 640 mA.
Figure 14. Inductor Current - Current Limit Operation
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