LTC3553 Linear Technology Corporation, LTC3553 Datasheet - Page 20

• Manufacturer Part Number: LTC3553
• Description: Micropower USB Power Manager
• Manufacturer: Linear Technology Corporation

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LTC3553
OPERATION
BUCK REGULATOR
Introduction
The LTC3553 includes a constant-frequency current-
mode 200mA buck regulator. At light loads, the regulator
automatically enters Burst Mode operation to maintain
high effi ciency.
Applications with a near-zero-current sleep or memory
keep-alive mode can command the LTC3553 buck regula-
tor into a standby mode that maintains output regulation
while drawing only 1.5μA quiescent current. Load capability
drops to 10mA in this mode.
The buck regulator is enabled, disabled and sequenced
through the pushbutton interface (see the Pushbutton
Interface section for more information). It is recommended
that the buck regulator input supply (BVIN) be connected
to the system supply pin (V
because the undervoltage lockout circuit on the V
(V
voltage drops below the V
ing the buck regulator input supply from a voltage other
than V
its specifi ed operating voltage range as operation is not
guaranteed beyond this range.
Output Voltage Programming
Figure 3 shows the buck regulator application circuit. The
output voltage for the buck regulator is programmed using
a resistor divider from the buck regulator output connected
to the feedback pin (BUCK_FB) such that:
Typical values for R1 can be as high as 2.2MΩ.
(R1 + R2) can be as high as 3MΩ. The capacitor C
cancels the pole created by feedback resistors and the
input capacitance of the BUCK_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
20
OUT
V
BUCK
UVLO) disables the buck regulator when the V
OUT
= 0.8V •
, the regulator should not be operated outside
⎛
⎜
⎝
R2
R1
+1
⎞
⎟
⎠
OUT
OUT
UVLO threshold. If driv-
). This is recommended
OUT
OUT
pin
FB
for C
applications. Experimentation with capacitor sizes between
2pF and 22pF may yield improved transient response.
Normal Buck Operating Mode (STBY Pin Low)
In normal mode (STBY pin low), the buck regulator per-
forms as a traditional constant-frequency current mode
switching regulator. Switching frequency is determined
by an internal oscillator which operates at 1.125MHz. An
internal latch is set at the start of every oscillator cycle,
turning on the main P-channel MOSFET switch. During
each cycle, a current comparator compares the inductor
current to the output of an error amplifi er. The output of
the current comparator resets the internal latch, which
causes the main P-channel MOSFET switch to turn off and
the N-channel MOSFET synchronous rectifi er to turn on.
The N-channel MOSFET synchronous rectifi er turns off
at the end of the clock cycle, or when the current through
the N-channel MOSFET synchronous rectifi er drops to
zero, whichever happens fi rst. Via this mechanism, the
error amplifi er adjusts the peak inductor current to deliver
the required output power. All necessary compensation
is internal to the buck regulator requiring only a single
ceramic output capacitor for stability.
At light load and no-load conditions, the buck automatically
switches to a power-saving hysteretic control algorithm
that operates the switches intermittently to minimize
switching losses. Known as Burst Mode operation, the
FB
STBY
EN
but a value of 10pF is recommended for most
Figure 3. Buck Regulator Application Circuit
CONTROL
PWM
GND
V
IN
0.8V
MP
MN
BUCK_FB
SW
L
C
FB
R1
R2
C
OUT
3553 F03
V
BUCK
3553f