LTC3554 LINER [Linear Technology], LTC3554 Datasheet - Page 20

• Manufacturer Part Number: LTC3554
• Description: Micropower USB Power Manager with Li-Ion Charger and Two Step-Down Regulators
• Manufacturer: LINER [Linear Technology]

OPERATION
LTC3554
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 effi ciency.
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 effi ciency 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
This is recommended because the undervoltage lockout
circuit on the V
down switching regulators when the V
20
OUT
pin (V
OUT
UVLO) disables the step-
OUT
FSEL
EN
Figure 3. Step-down Switching Regulator
Application Circuit
voltage drops
CONTROL
PWM
GND
OUT
).
V
IN
0.8V
MP
MN
SWx
FBx
below the V
switching regulator input supplies from a voltage other
than V
their specifi ed 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:
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 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
applications. Experimentation with capacitor sizes between
2pF and 22pF may yield improved transient response.
V
OUTx
FB
L
OUT
C
FB
but a value of 10pF is recommended for most
= 0.8V •
, the regulators should not be operated outside
OUT
R1
R2
UVLO threshold. If driving the step-down
R2
R1
C
+1
OUT
3554 F03
V
OUTx
3554f
FB