LTC3554 Datasheet, PDF (20/36 Pages) Linear Technology – Micropower USB Power Manager with Li-Ion Charger and Two Step-Down Regulators

LTC3554
OPERATION
STEP-DOWN SWITCHING REGULATOR
Introduction
The LTC3554 includes two constant-frequency cur-
rent-mode 200mA step-down switching regulators, also
known as buck regulators. At light loads, each regulator
automatically enters Burst Mode operation to maintain
high efficiency.
Applications with a near-zero-current sleep or memory
keep-alive mode can command the LTC3554 switching
regulators into a standby mode that maintains output
regulation while drawing only 1.5μA quiescent current
per active regulator. Load capability drops to 5mA per
regulator in this mode.
Switching frequency and switch slew rate are pin-select-
able, allowing the application circuit to dynamically trade
off efficiency and EMI performance.
The regulators are enabled, disabled and sequenced
through the pushbutton interface (see the Pushbutton
Interface section for more information). It is recommended
that the step-down switching regulator input supply
(BVIN) be connected to the system supply pin (V_OUT).
This is recommended because the undervoltage lockout
circuit on the V_OUT pin (V_OUT UVLO) disables the step-
down switching regulators when the V_OUT voltage drops
below the V_OUT UVLO threshold. If driving the step-down
switching regulator input supplies from a voltage other
than V_OUT, the regulators should not be operated outside
their specified operating voltage range as operation is not
guaranteed beyond this range.
Output Voltage Programming
Figure 3 shows the step-down switching regulator
application circuit. The output voltage for each step-down
switching regulator is programmed using a resistor divider
from the step-down switching regulator output connected
to the feedback pins (FB1 and FB2) such that:
V_OUTx
=
0.8V
•



R1
R2
+
1^
Typical values for R1 can be as high as 2.2MΩ.
(R1 + R2) can be as high as 3MΩ. The capacitor C_FB
cancels the pole created by feedback resistors 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_FB but a value of 10pF is recommended for most
applications. Experimentation with capacitor sizes between
2pF and 22pF may yield improved transient response.
EN
MODE
SLEW
PWM
CONTROL
GND
V_IN
MP
SWx
MN
FBx
0.8V
L
C_FB
V_OUTx
R1
C_OUT
R2
3554 F03
Figure 3. Step-down Switching Regulator
Application Circuit
3554p
20