Datasheet

LTC3447
8
3447f
the user should calculate the power dissipation when
the LTC3447 is used at 100% duty cycle with low input
voltage (See Thermal Considerations in the Applications
Information section).
Low Supply Operation
The LTC3447 will operate with input supply voltages as
low as 2.5V, but the maximum allowable output current
is reduced at this low voltage. Figure 4 shows the reduc-
tion in the maximum output current as a function of input
voltage for various output voltages.
Slope Compensation and Inductor Peak Current
Slope compensation provides stability in constant fre-
quency architectures by preventing subharmonic oscil-
lations at high duty cycles. It is accomplished internally
by adding a compensating ramp to the inductor current
signal at duty cycles in excess of 40%. Normally, this
results in a reduction of maximum inductor peak current
for duty cycles >40%; however, the LTC3447 uses a
patent-pending scheme that counteracts this compensat-
ing ramp, which allows the maximum inductor peak current
to remain unaffected throughout all duty cycles.
DAC
The I
2
C interface is used to control the internal voltage
DAC for the buck regulator. The output voltage range is
0.69V to 2.05V in 21.6mV steps. The default DAC setting
is 100000 which equates to a 1.38V output voltage. Output
voltage transitions begin once the I
2
C interface receives
the STOP command.
Slew Rate
The LTC3447 has a slew rate of approximately 11mV/µs.
The slew rate is controlled by the RC time constant of a
low pass fi lter at the voltage DAC output. Figure 5 shows
a typical transition from min to max DAC settings.
SUPPLY VOLTAGE (V)
2.5
MAXIMUM LOAD CURRENT (mA)
1200
1100
1000
900
800
700
3.0
3.5 4.0 4.5
3447 F04
5.0 5.5
DAC = MAX
DAC = MIN
T
A
= 25°C
Figure 6. Typical I
2
C Write Protocol
SDA
SCL
1-7 8 9 1-7 9 1-7 8 98
ADDRESS R/W ACK DATA ACK DATA ACK
START
COMMAND
ST0P
COMMAND
S P
3447 TD02
Figure 5.Transition from DAC = MIN to DAC = MAX
OPERATIO
U
Figure 4. Maximum Load Current vs Supply Voltage
BURST MODE OPERATION
V
IN
= 3.6V
V
OUT
INDUCTOR CURRENT
PGOOD
100µs/DIV
500mV/DIV
5V/DIV
200mA/DIV