LTC3809EMSE-1 Linear Technology, LTC3809EMSE-1 Datasheet - Page 9

• Manufacturer Part Number: LTC3809EMSE-1
• Description: IC CTRLR DC/DC SYNC 10-MSOP
• Manufacturer: Linear Technology
• Type: Step-Down (Buck)r

Specifications of LTC3809EMSE-1

• Internal Switch(s): No
• Synchronous Rectifier: Yes
• Number Of Outputs: 1
• Voltage - Output: 0.6 ~ 9.8 V
• Current - Output: 1A
• Frequency - Switching: 550kHz
• Voltage - Input: 2.75 ~ 9.8 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 10-MSOP Exposed Pad, 10-HMSOP, 10-eMSOP
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Power - Output: -

OPERATION
Main Control Loop
The LTC3809-1 uses a constant frequency, current mode
architecture. During normal operation, the top external
P-channel power MOSFET is turned on when the clock sets
the RS latch, and is turned off when the current comparator
(ICMP) resets the latch. The peak inductor current at which
ICMP resets the RS latch is determined by the voltage on the
I
(EAMP). The V
signal from an external resistor divider. This feedback
signal is compared to the internal 0.6V reference voltage
by the EAMP. When the load current increases, it causes a
slight decrease in V
in turn causes the I
inductor current matches the new load current. While the top
P-channel MOSFET is off, the bottom N-channel MOSFET is
turned on until either the inductor current starts to reverse,
as indicated by the current reversal comparator IRCMP , or
the beginning of the next cycle.
Shutdown, Soft-Start and Tracking Start-Up
(RUN and TRACK/SS Pins)
The LTC3809-1 is shut down by pulling the RUN pin low.
In shutdown, all controller functions are disabled and the
chip draws only 9μA. The TG output is held high (off) and
the BG output low (off) in shutdown. Releasing the RUN
pin allows an internal 0.7μA current source to pull up the
RUN pin to V
pin reaches 1.1V.
The start-up of V
nections on the TRACK/SS pin. The start-up of V
controlled by the LTC3809-1’s internal soft-start when
TRACK/SS is connected to V
amplifi er EAMP compares the feedback signal V
internal soft-start ramp (instead of the 0.6V reference),
which rises linearly from 0V to 0.6V in about 1ms. This al-
lows the output voltage to rise smoothly from 0V to its fi nal
value while maintaining control of the inductor current.
The 1ms soft-start time can be changed by connecting
the optional external soft-start capacitor C
TRACK/SS and GND pins. When the controller is enabled
TH
pin, which is driven by the output of the error amplifi er
IN
FB
. The controller is enabled when the RUN
OUT
pin receives the output voltage feedback
TH
FB
is based on the three different con-
voltage to increase until the average
relative to the 0.6V reference, which
(Refer to Functional Diagram)
IN
. During soft-start, the error
SS
between the
FB
OUT
to the
is
by releasing the RUN pin, the TRACK/SS pin is charged up
by an internal 1μA current source and rises linearly from
0V to above 0.6V. The error amplifi er EAMP compares the
feedback signal V
V
When the voltage on the TRACK/SS pin is less than the
0.6V internal reference, the LTC3809-1 regulates the V
voltage to the TRACK/SS pin instead of the 0.6V reference.
Therefore V
voltage V
V
of V
start-up, the regulated fi nal value of V
than that of V
same ratio as the divider on V
See detailed discussions in the Run and Soft-Start/Tracking
Functions in the Applications Information Section.
Light Load Operation (Burst Mode Operation,
Continuous Conduction or Pulse-Skipping Mode)
(MODE Pin)
The LTC3809-1 can be programmed for either high effi ciency
Burst Mode operation, forced continuous conduction mode
or pulse-skipping mode at low load currents. To select
Burst Mode operation, tie the MODE pin to V
forced continuous operation, tie the MODE pin to a DC
voltage below 0.4V (e.g., GND). Tying the MODE pin to a
DC voltage above 0.4V and below 1.2V (e.g., V
pulse-skipping mode. The 0.4V threshold between forced
continuous operation and pulse-skipping mode can be
used in secondary winding regulation as described in the
Auxiliary Winding Control Using the MODE Pin discussion
in the Applications Information section.
When the LTC3809-1 is in Burst Mode operation, the peak
current in the inductor is set to approximately one-fourth
of the maximum sense voltage even though the voltage on
the I
tor current is higher than the load current, the EAMP will
decrease the voltage on the I
drops below 0.85V, the internal SLEEP signal goes high
and the external MOSFET is turned off.
FB
X
is connected to the TRACK/SS pin to allow the start-up
OUT
linearly from 0V to 0.6V.
TH
to “track” that of V
pin indicates a lower value. If the average induc-
X
during start-up. Typically, a resistor divider on
OUT
OUT
of the LTC3809-1 can track an external
, and the resistor divider on V
FB
to this ramp instead, and regulates
X
. For coincident tracking during
TH
OUT
pin. When the I
that is connected to V
LTC3809-1
X
should be larger
IN
FB
. To select
TH
X
) enables
has the
voltage
38091fc
9
FB
FB
.