MAX8649 Maxim Integrated Products, MAX8649 Datasheet - Page 15

• Manufacturer Part Number: MAX8649
• Description: 1.8A Step-Down Regulator
• Manufacturer: Maxim Integrated Products

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The MAX8649 includes internal soft-start circuitry that
eliminates inrush current at startup, reducing transients
on the input source (see the Typical Operating Charac-
teristics ). Soft-start is particularly useful for high-imped-
ance input sources, such as Li+ and alkaline cells.
When enabling the MAX8649 into a prebiased output,
the MAX8649 performs a complete soft-start cycle.
An internal n-channel synchronous rectifier eliminates
the need for an external Schottky diode and improves
efficiency. The synchronous rectifier turns on during the
second half of each switching cycle (off-time). During
this time, the voltage across the inductor is reversed,
and the inductor current ramps down. In PWM mode,
the synchronous rectifier turns off at the end of the
switching cycle. In power-save mode, the synchronous
rectifier turns off when the inductor current falls below
50mA (typ) or at the end of the switching cycle,
whichever occurs first.
The MAX8649 output voltage has an actively controlled
variable ramp rate, set with the I
Figures 6, 7, and 8). The value set in the RAMP register
controls the output voltage ramp rate. The
RAMP_DOWN bit controls the active ramp-down
Figure 6. Ramp-Up Function
Figure 7. Ramp-Down Function
VOUT2
VOUT1
VOUT2
VOUT1
VOLTAGE
OUTPUT
VOLTAGE
OUTPUT
1.8A Step-Down Regulator with Differential
DELTA
V = 10mV
DELTA V = 10mV
______________________________________________________________________________________
Synchronous Rectification
10mV/RAMP
RATE
Ramp-Rate Control
Remote Sense in 2mm x 2mm WLP
2
C interface (see
10mV/RAMP RATE
TIME
TIME
Soft-Start
behavior in power-save mode. When the regulator is set
for power-save mode and the RAMP_DOWN bit is
cleared, the ramp-down is not actively controlled, and
the regulator output voltage ramps down at the rate
determined by the output capacitance and the external
load. Small loads result in an output-voltage decay that
is slower than that specified by RAMP; large loads
result in an output-voltage decay that is no faster than
that specified by RAMP When the RAMP_DOWN bit is
set in power-save mode, the zero-cross comparator is
disabled during the ramp-down condition. Active ramp-
down functionality is inherent in forced-PWM operation.
Calculate the maximum and minimum values for the
ramp rate as follows:
where:
f
f
f
n = 4 for 13MHz, 6 for 19.2MHz, and 8 for 26MHz
RAMP_CODE = value of the RAMP[2:0] register (see
Table 9)
Figure 8. Mode Change Before Final Value is Reached
SW
SW
SYNC
VOLTAGE
OUTPUT
FINAL
= 3.25MHz ±10% for PWM operation
= 3.25MHz ±25% for hysteretic operation
= frequency of external clock
t
t
RAMP MIN
RAMP
_
_ _
MAX
TO HIGHER VOUT
V
t
t
MODE CHANGE
CLK MAX
CLK MIN
OUT LSB
=
=
V
t
f
_
_
SW
CLK MAX
V
t
_
OUT LSB
CLK MIN
OUT LSB
=
_
_
_
=
_
f
=
=
SYNC
f
10
SW
f
n
SW MIN
mV
×
_ _ MAX
×
1
1
_
2
2
RAMP CODE
RAMP CODE
TO LOWER VOUT
MODE CHANGE
1
_
1
_
15