LM3102EVAL National Semiconductor, LM3102EVAL Datasheet - Page 12

• Manufacturer Part Number: LM3102EVAL
• Description: BOARD EVALUATION LM3102
• Manufacturer: National Semiconductor
• Series: PowerWise®, SIMPLE SWITCHER®r

Specifications of LM3102EVAL

• Main Purpose: DC/DC, Step Down
• Outputs And Type: 1, Non-Isolated
• Voltage - Output: 3.3V
• Current - Output: 2.5A
• Voltage - Input: 8 ~ 42V
• Regulator Topology: Buck
• Frequency - Switching: 500kHz
• Board Type: Fully Populated
• Utilized Ic / Part: LM3102
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Power - Output: -
• Lead Free Status / Rohs Status: Supplier Unconfirmed

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The LM3102 can be remotely shutdown by pulling the voltage
of the EN pin below 1V. In this shutdown mode, the SS pin is
internally grounded, the on-timer is disabled, and bias cur-
rents are reduced. Releasing the EN pin allows normal oper-
ation to resume because the EN pin is internally pulled up.
Current Limit
Current limit detection is carried out during the off-time by
monitoring the re-circulating current through the synchronous
MOSFET. Referring to the Functional Block Diagram, when
FIGURE 1. Shutdown Implementation
FIGURE 2. Inductor Current - Current Limit Operation
30021325
(5)
12
the main MOSFET is turned off, the inductor current flows
through the load, the PGND pin and the internal synchronous
MOSFET. If this current exceeds 2.7A, the current limit com-
parator toggles, and as a result disabling the start of the next
on-time period. The next switching cycle starts when the re-
circulating current falls back below 2.7A (and the voltage at
the FB pin is below 0.8V). The inductor current is monitored
during the on-time of the synchronous MOSFET. As long as
the inductor current exceeds 2.7A, the main MOSFET will re-
main inhibited to achieve current limit. The operating frequen-
cy is lower during current limit due to a longer off-time.
Figure 2 illustrates an inductor current waveform. On aver-
age, the output current I
current I
In case of current limit (the current limit portion of Figure 2),
the next on-time will not initiate until that the current drops
below 2.7A (assume the voltage at the FB pin is lower than
0.8V). During each on-time the current ramps up an amount
equal to:
During current limit, the LM3102 operates in a constant cur-
rent mode with an average output current I
2.7A + I
LR
L
, which is the average of the rippled inductor current.
/ 2.
OUT
is the same as the inductor
OUT(CL)
30021326
equal to
(6)