LTC3550-1
13
35501f
OPERATIO
U
600mA Step-Down Regulator
The LTC3550-1 regulator uses a constant frequency, cur-
rent mode step-down architecture. Both the top (P-channel
MOSFET) and bottom (N-channel MOSFET) switches are
internal. During normal operation, the internal top power
MOSFET is turned on each cycle when the oscillator sets
the RS latch, and is turned off when the current com-
parator, I
COMP
, resets the RS latch. The peak inductor
current at which I
COMP
resets the RS latch, is controlled
by the output of error amplifi er EA. When the load current
increases, it causes a slight decrease in the output voltage
(V
OUT
), relative to the internal reference, which in turn
causes the EA amplifi er’s output voltage to increase until
the average inductor current matches the new load cur-
rent. While the top MOSFET is off, the bottom MOSFET is
turned on until either the inductor current starts to reverse,
as indicated by the current reversal comparator I
RCMP
, or
the beginning of the next clock cycle.
Burst Mode
®
Operation
The LTC3550-1 buck regulator is capable of Burst Mode
operation in which the internal power MOSFETs operate
intermittently based on load current demand.
In Burst Mode operation, the peak current of the inductor is
set to approximately 200mA regardless of the output load.
Each burst event can last from a few cycles at light loads
to almost continuously cycling with short sleep intervals
at moderate loads. In between these burst events, the
power MOSFETs and any unneeded circuitry are turned
off, reducing the quiescent current to 20µA. In this sleep
state, the load current is being supplied solely from the
output capacitor. As the output voltage droops, the EA
amplifi er’s output rises above the sleep threshold signaling
the BURST comparator to trip and turn the top MOSFET
on. This process repeats at a rate that is dependent on
the load demand.
Dropout Operation
As the input supply voltage decreases to a value approach-
ing the output voltage, the duty cycle increases toward the
maximum on-time. Further reduction of the supply voltage
forces the main switch to remain on for more than one cycle
until it reaches 100% duty cycle. The output voltage will
then be determined by the input voltage minus the voltage
drop across the P-channel MOSFET and the inductor.
An important detail to remember is that at low input supply
voltages, the R
DS(ON)
of the P-channel switch increases
(see Typical Performance Characteristics). Therefore,
the user should calculate the power dissipation when the
LTC3550-1 is used at 100% duty cycle with low input
voltage (See Thermal Considerations in the Applications
Information section).
Battery Charger Power Source Selection
The LTC3550-1 can charge a battery from either the wall
adapter input or the USB port input. The charger automati-
cally senses the presence of voltage at each input. If both
power sources are present, the charger defaults to the wall
adapter source provided suffi cient power is present at the
DCIN input. “Suffi cient power” is defi ned as:
• Supply voltage is greater than the UVLO threshold.
• Supply voltage is greater than the battery voltage by
50mV (180mV rising, 50mV falling).
The open drain power status outputs (
⎯
P
⎯
W
⎯
R and USBPWR)
indicate which power source has been selected. Table 1
describes the behavior of these status outputs.
Table 1. Power Source Selection
V
USBIN
> 3.95V and
V
USBIN
> BAT + 50mV
V
USBIN
< 3.95V or
V
USBIN
< BAT + 50mV
V
DCIN
> 4.15V and
V
DCIN
> BAT + 50mV
Device Powered from
Wall Adapter Source;
USBIN Current < 25µA
⎯
P
⎯
W
⎯
R: LOW
USBPWR: LOW
Device Powered from
Wall Adapter Source
⎯
P
⎯
W
⎯
R: LOW
USBPWR: LOW
V
DCIN
< 4.15V or
V
DCIN
< BAT + 50mV
Device Powered from
USB Source;
⎯
P
⎯
W
⎯
R: LOW
USBPWR: Hi-Z
No Charging
⎯
P
⎯
W
⎯
R: Hi-Z
USBPWR: LOW
Status Indicators
The charge status output (
⎯
C
⎯
H
⎯
R
⎯
G) has two states: pull-
down and high impedance. The pull-down state indicates
that the LTC3550-1 is in a charge cycle. Once the charge
cycle has terminated or the LTC3550-1 is disabled, the
pin state becomes high impedance. The pull-down state
is strong enough to drive an LED and is capable of sink-
ing up to 10mA.
Burst Mode is a registered trademark of Linear Technology Corporation.
