Datasheet

LT3572
9
3572fa
APPLICATIONS INFORMATION
PWM
The LT3572 can PWM the output drivers at a very high
frequency. The limitation on the frequency is determined
by the internal rise in die temperature that occurs when
driving the motor. The power delivered to the piezo motor
is propotional to V
OUT
2
, the capacitance of the motor, and
the PWM frequency. When any of these are increased the
power dissipated in the part increases causing the internal
die temperature to increase. Driving two 2.2nF capacitors
with V
OUT
at 30V, the maximum PWM frequency should be
less than 80 kHz. The LT3572 can run at a higher frequency
but either V
OUT
needs to be reduced or the capacitance
needs to be lowered. A piezo motor has an associated
capacitance that cannot be reduced so the output voltage
must be lowered. Since the power is proportional to V
OUT
2
a
reduction of V
OUT
to 25V from 30V will allow the LT3572 to
run at a maxim frequency of 115 kHz. If a different motor is
used the maximum PWM frequency will need to be adjusted
inversely to the equivolent capacitance of the motor.
Inductor Selection
A 10μH inductor is recommended for most LT3572 ap-
plications. Choose an inductor that will handle at least
1A without saturating, and ensure that the inductor has a
low DCR (copper-wire resistance) to minimize I
2
R power
losses. Table 1 lists several inductor manufacturers.
Table 1. Inductor Manufacturers
Sumida [phone] www.sumida.com
TDK [phone] www.tdk.com
Murata [phone] www.murata.com
FDK [phone] www.tdk.co.jp
Capacitor Selection
The small size of ceramic capacitors makes them ideal
for LT3572 applications. Only X5R or X7R types should
be used because they retain their capacitance over wider
voltage and temperature ranges than other types such as
Y5V or Z5U. A 4.7μF to 15μF output capacitor is suffi cient
for stable transient response, however, more output ca-
pacitance can help limit the voltage droop on V
OUT
during
transients.
Ceramic capacitors also make a good choice for the input
decoupling capacitor, which should be placed as close as
possible to the LT3572. A 1μF to 4.7μF input capacitor
is suffi cient for most applications. Table 2 shows a list
1
20 19 18 17 16
6 7 8
10
2
3
4
5
15
14
13
12
11
9
R4
PWMB
PWMA
V
OUT
3572 BD LAYOUT
SHDN
SHDNA
SHDNB
FB
SS
C2
R2
R1
C
FF
PGOOD
OUTB
OUTB
OUTA
OUTA
SW
SYNC
RT
GND
GND
V
IN
C1
D1
L1
C3