LTC1700EMS#TR Linear Technology, LTC1700EMS#TR Datasheet - Page 7

• Manufacturer Part Number: LTC1700EMS#TR
• Description: IC DC/DC CONTRLR STP-UP 10-MSOP
• Manufacturer: Linear Technology
• Type: Step-Up (Boost)r

Specifications of LTC1700EMS#TR

• Internal Switch(s): No
• Synchronous Rectifier: Yes
• Number Of Outputs: 1
• Current - Output: 1A
• Frequency - Switching: 530kHz
• Voltage - Input: 0.9 ~ 6 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 10-MSOP, Micro10™, 10-uMAX, 10-uSOP
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Voltage - Output: -
• Power - Output: -
• Other names: LTC1700EMSTR

OPERATIO
output feedback voltage V
creases, it causes a slight decreases in V
1.205V reference, which in turn causes the I
increase until the average inductor current matches the
new load current.
The internal oscillator can be synchronized to an external
clock applied to the SYNC/MODE pin and can lock to a
frequency between 400kHz to 750kHz. When not synchro-
nized, the oscillator runs at 530kHz.
The main control loop is shut down by pulling the RUN/SS
pin low. Releasing the RUN/SS pin allows an internal
3.8µA current source to charge up an external soft-start
capacitor (C
control loop is enabled with the I
approximately 5% of its maximum value. As C
ues to charge, I
operation to resume.
An overvoltage comparator 0V guards against transient
overshoots greater than 5% above regulated voltage by
turning off both the external MOSFETs and keeping them
off until the fault is removed.
To prevent excessive inductor current buildup, the main
N-channel MOSFET is only allowed to turn on for a
maximum duty cycle of 90%.
Burst Mode Operation
The LTC1700 can be enabled to go into Burst Mode
operation at low load currents simply by connecting the
SYNC/MODE pin to a voltage of at least 1.2V. In this mode,
the peak current of the inductor is set as if V
low duty cycles) even though the voltage at the I
actually at a lower value. If the inductor’s average current
is greater than the load requirement, the voltage at the I
pin will drop. When the I
internal sleep signal goes low, turning off both external
MOSFETs. Now the load current will solely be supplied by
the output capacitor and the output voltage begins to
droop. This drooping of the output voltage results in the
rise of I
switching will then be resumed on the next oscillator cycle.
TH
voltage and once it has risen above 0.22V,
SS)
. When this voltage reaches 0.8V, the main
TH
U
is gradually released allowing normal
TH
FB
voltage goes below 0.12V, the
. When the load current in-
TH
voltage clamped at
FB
ITH
relative to the
TH
= 0.36V (at
SS
voltage to
TH
contin-
pin is
TH
Frequency Synchronization
The LTC1700 can be externally driven by a CMOS
(0V to 1.2V) compatible clock signal between 400kHz and
750kHz. Do not synchronize the LTC1700 below 400kHz
or above 750kHz as this may cause abnormal operation.
During synchronization, Burst Mode operation is
inhibited.
Low Input Operation
When the voltage at V
operates in the “start-up” mode. In this mode, most
internal circuitry is turned off except the start-up oscilla-
tor, current comparator (I
tor (SC). The voltage at pins TG and BG are forced to
ensure both the external MOSFETs are off. The start-up
oscillator runs at about 210kHz at 50% duty cycle and is
used to set the latch (L1) which turns on the internal
MOSFET M1 (see Functional Diagram). When the inductor’s
current reaches 60mA, the current comparator (I
tripped and resets the latch. This turns M1 off and the
parasitic diode of the external P-channel MOSFET is used
to transfer the energy from the inductor to the output
capacitor. The above cycle repeats again on the next
oscillator pulse.
When the output voltage rises above 2.3V, the start-up
comparator will trip, powering up the rest of the LTC1700.
All start-up circuitry will then be turned off. Now the
LTC1700 has successfully transitioned out of its start-up
mode and commences normal operation as described
under the section “Main Control Loop.”
Protection Circuitry
Two protection circuits are incorporated into the LTC1700.
To prevent the inductor from saturating the maximum
duty cycle of the regulator is limited to 90%. This is done
to ensure that at least 10% of the time energy is being
transferred from the inductor to the output capacitor.
Output overvoltage protection is also provided. Should the
output rises about 5% above the regulated value, both the
external MOSFETs will be forced off.
OUT
CMP
is less than 2.3V, the LTC1700
) and the start-up compara-
LTC1700
CMP
1700fa
7
) is