LT1374IS8-SYNC Linear Technology, LT1374IS8-SYNC Datasheet - Page 7

• Manufacturer Part Number: LT1374IS8-SYNC
• Description: IC SW REG 4.5A ADJ STP-DWN 8SOIC
• Manufacturer: Linear Technology
• Type: Step-Down (Buck)r

Specifications of LT1374IS8-SYNC

• Internal Switch(s): Yes
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Voltage - Output: 2.42 ~ 21.5 V
• Current - Output: 4.5A
• Frequency - Switching: 500kHz
• Voltage - Input: 5 ~ 25 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 8-SOIC (3.9mm Width)
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Power - Output: -

BLOCK DIAGRA
PI FU CTIO S
The LT1374 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 oscilla-
tor pulse which sets the R
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
SYNC: (Excludes T7 package) 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 synchroniz-
ing range is equal to initial operating frequency, up to
1MHz. This pin replaces SHDN on -SYNC option parts. See
Synchronizing section in Applications Information for
details.
SHDN: 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 thresholds, 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 can be used to prevent the regulator from operating
until the input voltage has reached a predetermined level.
V
input of the peak switch current comparator. It is normally
C
: The V
U
C
U
pin is the output of the error amplifier and the
U
S
W
flip-flop to turn the switch on.
used for frequency compensation, but can do double duty
as a current clamp or control loop override. This pin sits
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.
BIAS: (SO-8 and FE16 Packages) The BIAS pin is used to
improve efficiency when operating at higher input volt-
ages and light load current. Connecting this pin to the
regulated output voltage forces most of the internal cir-
cuitry to draw its operating current from the output voltage
rather than the input supply. This is a much more efficient
way of doing business if the input voltage is much higher
than the output. Minimum output voltage setting for this
mode of operation is 3.3V . Efficiency improvement at
V
NC: No Connect. Leave floating or solder to any node.
it much easier to frequency compensate the feedback loop
and also gives much quicker transient response.
Most of the circuitry of the LT1374 operates from an
internal 2.9V bias line. The bias regulator normally draws
power from the regulator input pin, but if the BIAS pin is
connected to an external voltage higher than 3V, bias
power will be drawn from the external source (typically the
regulated output voltage). This will improve efficiency if
the BIAS pin voltage is lower than regulator input voltage.
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 saturate. This
boosted voltage is generated with an external capacitor
and diode. Two comparators are connected to the shut-
down pin. One has a 2.38V threshold for undervoltage
lockout and the second has a 0.4V threshold for complete
shutdown.
IN
= 20V, V
OUT
= 5V, and I
OUT
= 25mA is over 10%.
LT1374
1374fd
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