LTC3555 Linear Technology, LTC3555 Datasheet - Page 23

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

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The step-down switching regulators allow mode transition
on the fl y, providing seamless transition between modes
even under load. This allows the user to switch back and
forth between modes to reduce output ripple or increase
low current effi ciency as needed.
Step-Down Switching Regulator in Shutdown
The step-down switching regulators are in shutdown when
not enabled for operation. In shutdown, all circuitry in
the step-down switching regulator is disconnected from
the switching regulator input supply leaving only a few
nanoamperes of leakage current. The step-down switch-
ing regulator outputs are individually pulled to ground
through a 10k resistor on the switch pins (SW1-SW3)
when in shutdown.
General Purpose Switching Regulator Dropout
Operation
It is possible for a switching regulator’s input voltage,
V
battery voltage of 3.4V with a programmed output voltage
of 3.3V). When this happens, the PMOS switch duty cycle
increases until it is turned on continuously at 100%. In this
dropout condition, the respective output voltage equals the
regulator’s input voltage minus the voltage drops across
the internal P-channel MOSFET and the inductor.
CLPROG Resistor and Capacitor
As described in the High Effi ciency Switching PowerPath
Controller section, the resistor on the CLPROG pin deter-
mines the average input current limit when the switching
regulator is set to either the 1x mode (USB 100mA), the
5x mode (USB 500mA) or the 10x mode. The input cur-
rent will be comprised of two components, the current
that is used to drive V
switching regulator. To ensure that the USB specifi cation
is strictly met, both components of input current should
be considered. The Electrical Characteristics table gives
values for quiescent currents in either setting as well as
OPERATION
APPLICATIONS INFORMATION
INx
, to approach its programmed output voltage (e.g., a
OUT
and the quiescent current of the
Step-Down Switching Regulator Soft-Start Operation
Soft-start is accomplished by gradually increasing the
peak inductor current for each switching regulator over a
500μs period. This allows each output to rise slowly, help-
ing minimize the battery surge current. A soft-start cycle
occurs whenever a given switching regulator is enabled,
or after a fault condition has occurred (thermal shutdown
or UVLO). A soft-start cycle is not triggered by changing
operating modes. This allows seamless output operation
when transitioning between Forced Burst Mode, Burst
Mode, Pulse Skip mode or LDO operation.
Step-Down Switching Regulator Switching Slew Rate
Control
The step-down switching regulators contain new patent
pending circuitry to limit the slew rate of the switch node
(SWx). This new circuitry is designed to transition the switch
node over a period of a couple of nanoseconds, signifi cantly
reducing radiated EMI and conducted supply noise.
Low Supply Operation
The LTC3555 incorporates an undervoltage lockout circuit
on V
regulators when V
prevents unstable operation.
current limit programming accuracy. To get as close to
the 500mA or 100mA specifi cations as possible, a 1%
resistor should be used. Recall that I
V
An averaging capacitor or an R-C combination is required
in parallel with the CLPROG resistor so that the switching
regulator can determine the average input current. This
network also provides the dominant pole for the feedback
loop when current limit is reached. To ensure stability,
the capacitor on CLPROG should be 0.47μF or larger.
Alternatively, faster transient response may be achieved
with 0.1μF in series with 8.2Ω.
CLPROG
OUT
which shuts down the general purpose switching
/R
CLPPROG
OUT
• (h
drops below V
CLPROG
+1).
OUTUVLO
LTC3555
VBUS
. This UVLO
= I
VBUSQ
23
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