LTC3555 Linear Technology, LTC3555 Datasheet - Page 21

• Manufacturer Part Number: LTC3555
• Description: High Efficiency USB Power Manager + Triple Step-Down DC/DC
• Manufacturer: Linear Technology

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OPERATION
such as the programmability of switching regulators 2
and 3’s output voltage and the battery charger disable
feature. In this mode, both of the programmable switching
regulators have a fi xed servo voltage of 0.8V.
Because the SDA and SCL pins have no other context when
DV
the switching regulator mode bits B5 and B6. SCL maps
to B5 and SDA maps to B6.
⎯ R ⎯ S ⎯ T ⎯ 3 Pin
The ⎯ R ⎯ S ⎯ T ⎯ 3 pin is an open-drain output used to indicate that
switching regulator 3 has been enabled and has reached
its fi nal voltage. ⎯ R ⎯ S ⎯ T ⎯ 3 remains low impedance until regula-
tor 3 reaches 92% of its regulation value. A 230ms delay is
included to allow a system microcontroller ample time to
reset itself. ⎯ R ⎯ S ⎯ T ⎯ 3 may be used as a power-on reset to the
microprocessor powered by regulator 3 or may be used to
enable regulators 1 and/or 2 for supply sequencing. ⎯ R ⎯ S ⎯ T ⎯ 3
is an open-drain output and requires a pull-up resistor to
the output voltage of regulator 3 or another appropriate
power source
General Purpose Step-Down Switching Regulators
The LTC3555 contains three general purpose 2.25MHz
step-down constant-frequency current mode switching
regulators. Two regulators provide up to 400mA and a
third switching regulator can produce up to 1A. All three
switching regulators can be programmed for a minimum
output voltage of 0.8V and can be used to power a micro-
controller core, microcontroller I/O, memory, disk drive or
other logic circuitry. Two of the switching regulators have
I
All three converters support 100% duty cycle operation
(low dropout mode) when their input voltage drops very
close to their output voltage. To suit a variety of applica-
tions, selectable mode functions can be used to trade-off
noise for effi ciency. Four modes are available to control
the operation of the LTC3555’s general purpose switching
regulators. At moderate to heavy loads, the Pulse Skip
mode provides the least noise switching solution. At lighter
loads, either Burst Mode operation, Forced Burst Mode
operation or LDO mode may be selected. The switching
regulators include soft-start to limit inrush current when
powering on, short-circuit current protection and switch
2
C programmable set-points for on-the-fl y power savings.
CC
is grounded, these pins are re-mapped to control
node slew limiting circuitry to reduce radiated EMI. No
external compensation components are required. The
operating mode of the regulators may be set by either I
control or by manual control of the SDA and SCL pins if
the I
ally enabled by either their external control pins EN1, EN2,
EN3 or by the I
individual programmable feedback servo voltages via I
control. The switching regulator input supplies V
and V
pin V
Step-Down Switching Regulator Output Voltage
Programming
All three switching regulators can be programmed for
output voltages greater than 0.8V. Switching regulators 2
and 3 have I
has a single fi xed set-point. The full-scale output voltage for
each switching regulator is programmed using a resistor
divider from the switching regulator output connected to
the feedback pins (FB1, FB2 and FB3) such that:
where V
regulators 2 and 3 and V
regulator 1. See Figure 4
Typical values for R1 are in the range of 40k to 1M. The
capacitor C
tors and the input capacitance of the FB pin and also helps
to improve transient response for output voltages much
greater than 0.8V. A variety of capacitor sizes can be used
for C
plications. Experimentation with capacitor sizes between
2pF and 22pF may yield improved transient response.
V
2
OUTX
FB
OUT
C port is not used. Each converter may be individu-
IN3
but a value of 10pF is recommended for most ap-
.
FBX
will generally be connected to the system load
Figure 4. Buck Converter Application Circuit
=
FB
2
V
LTC3555
C programmable set-points while regulator 1
ranges from 0.425V to 0.8V for switching
FBX
cancels the pole created by feedback resis-
V
2
INx
C port. Switching regulators 2 and 3, have
SWx
GND
⎛
⎝ ⎜
FBx
R
R
2
1
+
1
L
FBX
C
⎞
⎠ ⎟
FB
is fi xed at 0.8V for switching
R1
R2
3555 F04
LTC3555
C
OUT
V
OUTx
IN1
21
, V
3555f
IN2
2
2
C
C