LTC3442 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
LTC3442
Linear Technology
'LTC3442' PDF : 20 Pages View PDF
LTC3442
APPLICATIONS INFORMATION
Optional Schottky Diodes
The Schottky diodes across the synchronous switches B
and D are not required (VOUT < 4.3V), but provide a lower
drop during the break-before-make time (typically 15ns)
improving efficiency. Use a surface mount Schottky diode
such as an MBRM120T3 or equivalent. Do not use ordi-
nary rectifier diodes, since the slow recovery times will
compromise efficiency. For applications with an output
voltage above 4.3V, a Schottky diode is required from
SW2 to VOUT.
Output Voltage < 2.4V
The LTC3442 can operate as a buck converter with out-
put voltages as low as 0.4V. The part is specified at 2.4V
minimum to allow operation without the requirement of a
Schottky diode. Synchronous switch D is powered from
VOUT and the RDS(ON) will increase at low output voltages,
therefore a Schottky diode is required from SW2 to VOUT
to provide the conduction path to the output. Note that
Burst Mode operation is inhibited at output voltages below
1.6V typical.
Output Voltage > 4.3V
A Schottky diode from SW2 to VOUT is required for output
voltages over 4.3V. The diode must be located as close to
the pins as possible in order to reduce the peak voltage
on SW2 due to the parasitic lead and trace inductance.
Input Voltage > 4.5V
For applications with input voltages above 4.5V which
could exhibit an overload or short-circuit condition, a
2Ω/1nF series snubber is required between SW1 and GND.
A Schottky diode from SW1 to VIN should also be added
as close to the pins as possible. For the higher input volt-
ages, VIN bypassing becomes more critical; therefore, a
ceramic bypass capacitor as close to the VIN and SGND
pins as possible is also required.
Operating Frequency Selection
Higher operating frequencies allow the use of a smaller
inductor and smaller input and output filter capaci-
tors, thus reducing board area and component height.
However, higher operating frequencies also increase the
IC’s total quiescent current due to the gate charge of the
four switches, as given by:
Buck:
Iq = (0.8 • VIN • f) mA
Boost:
Iq = [0.4 • (VIN + VOUT) • f] mA
Buck/Boost: Iq = [f • (1.2 • VIN + 0.4 • VOUT)] mA
where f = switching frequency in MHz. Therefore frequency
selection is a compromise between the optimal efficiency
and the smallest solution size.
Closing the Feedback Loop
The LTC3442 incorporates voltage mode PWM control. The
control to output gain varies with operation region (buck,
boost, buck/boost), but is usually no greater than 15. The
output filter exhibits a double pole response, as given by:
fFILTER—POLE = 2 • π •
1
Hz
L • COUT
(in buck mode)
f FILTER —POLE
=
2
•
VOUT
•
VIN
π•
Hz
L • COUT
(in boost mode)
where L is in henries and COUT is in farads.
The output filter zero is given by:
fFILTER—ZERO =
1
2 • π • RESR
• COUT
Hz
where RESR is the equivalent series resistance of the
output capacitor.
A troublesome feature in boost mode is the right-half plane
zero (RHP), given by:
fRHPZ
=
2
•
π
VIN2
• IOUT • L
•
VOUT
Hz
The loop gain is typically rolled off before the RHP zero
frequency.
14
For more information www.linear.com/LTC3442
3442fb
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