LM3460M5-1.2 National Semiconductor, LM3460M5-1.2 Datasheet - Page 8

• Manufacturer Part Number: LM3460M5-1.2
• Description: IC CONTROLLER PRECISION SOT23-5
• Manufacturer: National Semiconductor

Specifications of LM3460M5-1.2

• Controller Type: Precision Controller
• Voltage - Supply: 3 V ~ 3.6 V
• Current - Supply: 85mA
• Operating Temperature: 0°C ~ 70°C
• Mounting Type: Surface Mount
• Package / Case: SOT-23-5, SC-74A, SOT-25
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Interface: -
• Other names: LM3460M5-1.2TR

www.national.com
App Circuit Technical Information
In the circuit of Figure 2, drive for Q1 is provided by R7, but
only when U1 sources current: The operation of the drive
scheme is as follows:
If the voltage at V
the OUT pin of U1. Q1 is held OFF as the current flowing
down through R7 goes through D1 and R5 to ground.
IMPORTANT: Diode D1 is a 1N4001 because its V
much less than the V
not work here).
When U1 is not sourcing current, the voltage at the OUT pin
(and the cathode of D1) will be held at about 50 mV by the
R7/D1/R5 divider. The current flowing to ground through
these components is about 110 µA.
(Continued)
OUT
BE
is below 1.2V, no current flows from
of Q1 (a signal diode like 1N4148 will
F
must be
8
Because D1 is a 1A power diode, the V
small value of current will be much less than the V
to turn ON Q1 (so Q1 is held off by D1).
When U1 begins to source current (to cut off the pass
transistor and regulate V
cathode of D1 to rise.
This action causes the current that was flowing through D1
to flow into the base of Q1, turning it ON and taking drive
away from the base of Q2.
This action provides the negative feedback required to regu-
late V
of total supply voltage across the device.
OUT
and allows the LM3460 to operate with only 1.2V
OUT
) it forces the voltage at the
F
across D1 at this
BE
needed