lt3845ife-trpbf Linear Technology Corporation, lt3845ife-trpbf Datasheet - Page 7

• Manufacturer Part Number: lt3845ife-trpbf
• Description: High Voltage Synchronous Current Mode Step-down Controller With Adjustable Operating Frequency
• Manufacturer: Linear Technology Corporation

threshold, which limits the negative excursion of the pin
voltage. Therefore, this pin cannot be pulled low with a
low impedance source. If the V
manipulated, do so through a 1kΩ series resistance.
SYNC (Pin 7): The Sync pin provides an external clock
input for synchronization of the internal oscillator. R
is set such that the internal oscillator frequency is 10%
to 25% below the external clock frequency. If unused the
Sync pin is connected to SGND. For more information see
“Oscillator Sync” in the Application Information section
of this datasheet.
f
with an external resistor, R
even when supplying external sync clock signal. See the
Applications Information section for resistor value selec-
tion details.
SENSE
the current sense amplifi er and is connected to the V
side of the sense resistor for step-down applications. The
sensed inductor current limit is set to ±100mV across the
SENSE inputs.
SENSE
the current sense amplifi er and is connected to the induc-
tor side of the sense resistor for step-down applications.
The sensed inductor current limit is set to ±100mV across
the SENSE inputs.
PGND (Pin 11): The PGND pin is the high-current ground
reference for internal low side switch driver and the V
regulator circuit. Connect the pin directly to the negative
terminal of the V
tion Information section for helpful hints on PCB layout
of grounds.
BG (Pin 12): The BG pin is the gate drive for the bottom
N-channel MOSFET. Since very fast high currents are driven
from this pin, connect it to the gate of the power MOSFET
with a short and wide, typically 0.02" width, PCB trace to
minimize inductance.
PIN FUNCTIONS
SET
(Pin 8): The f
–
+
(Pin 10): The SENSE
(Pin 9): The SENSE
CC
SET
decoupling capacitor. See the Applica-
pin programs the oscillator frequency
SET
–
+
pin is the negative input for
pin is the positive input for
. The resistor is required
C
pin must be externally
OUT
SET
CC
V
decoupling node. Use a low ESR, 1μF or greater ceramic
capacitor to decouple this node to PGND. Most internal IC
functions are powered from this bias supply. An external
diode connected from V
bootstrapped capacitor during the off-time of the main
power switch. Back driving the V
DC voltage source, such as the V
tor supply, increases overall effi ciency and reduces power
dissipation in the IC. In shutdown mode this pin sinks
20μA until the pin voltage is discharged to 0V.
SW (Pin 14): Reference for V
rent Return for Bootstrapped Switch.
TG (Pin 15): The TG pin is the bootstrapped gate drive
for the top N-Channel MOSFET. Since very fast high cur-
rents are driven from this pin, connect it to the gate of
the power MOSFET with a short and wide, typically 0.02”
width, PCB trace to minimize inductance.
BOOST (Pin 16): The BOOST pin is the supply for the
bootstrapped gate drive and is externally connected to a
low ESR ceramic boost capacitor referenced to SW pin.
The recommended value of the BOOST capacitor,C
is at least 50 times greater than the total gate capacitance
of the topside MOSFET. In most applications 0.1μF is
adequate. The maximum voltage that this pin sees is V
+ V
SGND (Pin 17): The SGND pin is the low noise ground
reference. It should be connected to the –V
output capacitors. Careful layout of the PCB is necessary
to keep high currents away from this SGND connection.
See the Application Information section for helpful hints
on PCB layout of grounds.
Exposed Pad (SGND) (Pin 17): The exposed leadframe is
internally connected to the SGND pin. Solder the exposed
pad to the PCB ground for electrical contact and optimal
thermal performance.
CC
CC
(Pin 13): The V
, ground referred.
CC
CC
pin is the internal bias supply
to the BOOST pin charges the
BOOST
OUT
CC
Supply and High Cur-
pin from an external
output of the regula-
OUT
LT3845
side of the
BOOST
3845fc
7
IN
,