6
LT1507
PIN FUNCTIONS
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saturation voltage and maximum duty cycle. A typical
value for minimum input voltage is 1V above output
voltage. Start-up conditions may require more voltage at
light loads. See Minimum Input Voltage for details.
V
SW
(Pin 3): The switch pin is driven up to the input voltage
in the ON state and is an open circuit in the OFF state. At
higher load currents, pin voltage during the off condition
will be one diode drop below ground as set by the external
catch diode. At lighter loads the pin will assume an
intermediate state equal to output voltage during part of
the switch OFF time. Maximum
negative
voltage on the
switch pin is 1V with respect to the GND pin, so it must
always be clamped with a catch diode to the GND pin.
SHDN (Pin 4): The shutdown pin is used to turn off the
regulator and to reduce input drain current to a few
microamperes. Actually this pin has two separate thresh-
olds, one at 2.38V to disable switching and a second at
0.4V to force complete micropower shutdown. The 2.38V
threshold functions as an accurate undervoltage lockout
(UVLO). This is sometimes used to prevent the regulator
from delivering power until the input voltage has reached
a predetermined level.
BLOCK DIAGRAM
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The LT1507 is a constant frequency, current mode buck
converter. This means that there is an internal clock and
two feedback loops that control the duty cycle of the power
switch. In addition to the normal error amplifier, there is
a current sense amplifier that monitors switch current on
a cycle-by-cycle basis. A switch cycle starts with an
oscillator pulse which sets the RS flip-flop to turn the
switch on. When switch current reaches a level set by the
inverting input of the comparator, the flip-flop is reset and
the switch turns off. Output voltage control is obtained by
using the output of the error amplifier to set the switch
current trip point. This technique means that the error
amplifier commands current to be delivered to the output
rather than voltage. A voltage fed system will have low
phase shift up to the resonant frequency of the inductor
and output capacitor, then an abrupt 180° shift will occur.
The current fed system will have 90° phase shift at a much
lower frequency, but will not have the additional 90° shift
until well beyond the LC resonant frequency. This makes
it much easier to frequency compensate the feedback loop
and also gives much quicker transient response.
High switch efficiency is attained by using the BOOST pin
to provide a voltage to the switch driver which is higher
than the input voltage, allowing the switch to be saturated.
This boosted voltage is generated with an external capaci-
tor and diode.
Two comparators are connected to the shutdown pin. One
has a 2.38V threshold for undervoltage lockout and the
second has a 0.4V threshold for complete shutdown.
SYNC (Pin 5): The SYNC pin is used to synchronize the
internal oscillator to an external signal. It is directly logic
compatible and can be driven with any signal between
10% and 90% duty cycle. The synchronizing range is
equal to
initial
operating frequency up to 1MHz. See
Sychronizing section for details.
FB/SENSE (Pin 7): The feedback pin is used to set output
voltage using an external voltage divider that generates
2.42V at the pin with the desired output voltage. The fixed
voltage (– 3 .3V) parts have the divider included on the chip
and the feedback pin is used as a sense pin connected
directly to the 5V output. Two additional functions are
performed by the feedback pin. When the pin voltage
drops below 1.7V, switch current limit is reduced. Below
1V, switching frequency is also reduced. See More Than
Just Voltage Feedback.
V
C
(Pin 8): The V
C
pin is the output of the error amplifier
and the input of the peak switch current comparator. It is
normally used for frequency compensation but can do
double duty as a current clamp or control loop override.
This pin sets at about 1V for very light loads and 2V at
maximum load. It can be driven to ground to shut off the
regulator, but if driven high, current must be limited to 4mA.
