LT1500 데이터 시트보기 (PDF) - Linear Technology
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LT1500 Datasheet PDF : 16 Pages
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LT1500/LT1501
PIN FUNCTIONS
SHDN: Logic Level Shutdown Pin. This pin must be held
high (> 1.1V) for the regulator to run. SHDN can be tied
directly to VIN, even with VIN = 18V. The low-battery
detector remains active in shutdown, but all other circuitry
is turned off.
VIN: This pin supplies power to the regulator and is
connected to one side of the inductor sense resistor. It
should be bypassed close to the chip with a low ESR
capacitor.
ISENSE: This is one end of the internal inductor-current
sense resistor. With most applications, only the external
inductor is tied to this pin.
GND: This pin carries only low level current in the LT1500,
but it carries full switch current in the LT1501. The
negative end of the input bypass capacitor should be
connected close to this pin and the pin should go directly
to the ground plane with the LT1501.
PGND (LT1500 Only): This pin is the emitter of the internal
NPN power switch. Connect it directly to the ground plane.
SW: This is the collector of the internal NPN power switch.
To avoid EMI and overvoltage spikes, keep connections to
this pin very short.
LBI: This is the input to the low-battery detector with a
threshold of 1.24V. Maximum pin voltage is 5V. Bypass
LBI with a small filter capacitor when used. If unused, tie
LBI to ground. The low-battery detector remains active in
shutdown.
LBO : This is the open collector output of the low-battery
detector. It will sink up to 2mA. Leave open if not used.
FB/VOUT: FB is the inverting input to the error amplifier with
a regulating point of 1.265V and a typical bias current of
30nA. Bias current is reduced with a canceling circuit, so
bias current could flow in either direction. FB is replaced
with VOUT on fixed voltage parts. VOUT is the top of an
internal divider that is connected to the internal FB node.
A switch disconnects the divider in shutdown so that the
divider current does not load VIN through the inductor and
catch diode.
NFB/SELECT (LT1500 Only): NFB is a second feedback
node used to regulate a negative output voltage. Negative
output voltages can be generated by using a transformer
flyback circuit, a Cuk converter or a capacitor charge pump
added to a boost converter. The regulating point for NFB
is 1.265V and the internal resistance to ground is 100kΩ.
External divider current should be 300µA or greater to
avoid negative output voltage variations due to production
variations in the internal resistor value. FB should be left
open when using NFB.
On fixed voltage parts, NFB is replaced with Select. The
Select pin is used to set output voltage at either 3.3V or 5V.
VC (LT1500 Only): This is the output of the error amplifier
and the input to the current comparator. The VC pin voltage
is about 700mV at very light loads and about 1.2V at full
load. An internal comparator detects when the VC voltage
drops below about 750mV and shuts down the current
comparator and the power switch biasing to reduce quies-
cent current. This forces the regulator to operate in Burst
Mode operation.
SYNC (LT1500 Only): This is a logic level input used to
synchronize switching frequency to an external clock. The
sync signal overrides the internal current comparator and
turns the switch on. Minimum sync pulse width should be
50ns and maximum width should be 300ns. A continuous
high sync signal will force the power switch to stay on
indefinitely and current will increase without limit. Don’t
do this!
SS (LT1500 Only): This is the soft start function using the
base of a PNP transistor whose emitter is tied to the VC pin.
Grounding SS will turn off switching by pulling VC low. A
capacitor tied from SS to ground will force VC to ramp up
slowly during start-up at a rate set by the capacitor value
and the internal 4µA pull-up current. An external resistor
must be used to reset the capacitor voltage completely to
0V at power down.
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