LTC3527/LTC3527-1
10
35271fc
OPERATION
The LTC3527/LTC3527-1 are dual 1.2MHz/2.2MHz syn-
chronous boost converters housed in a 16-lead 3mm ×
3mm QFN package. With the ability to start up and operate
from inputs less than 880mV, these devices feature fi xed
frequency, current mode PWM control for exceptional line
and load regulation. The current mode architecture with
adaptive slope compensation provides excellent transient
load response, requiring minimal output fi ltering. Internal
soft-start and loop compensation simplifi es the design
process while minimizing the number of external com-
ponents. Each converter has a separate input supply pin
and is operated independently of the other, but they share
the same oscillator thus providing in-phase switching.
If different input supply voltages are used, the third V
IN
pin must be wired to the higher of the two supplies and
each V
OUT
must be higher than the highest V
IN
. Bypass
capacitors are recommended on all V
IN
pins.
With low R
DS(ON)
and low gate charge internal N-channel
MOSFET switches and P-channel MOSFET synchronous
rectifi ers, the LTC3527/LTC3527-1 achieve high effi ciency
over a wide range of load current. With the MODE pin low,
automatic Burst Mode operation maintains high effi ciency
at very light loads, reducing the quiescent current to just
12μA. If MODE is high, fi xed frequency PWM switching
provides low voltage ripple on the outputs. Operation can
be best understood by referring to the Block Diagram.
A PGOOD signal is provided independently for each con-
verter which can be used with the SHDN pins to provide
sequencing of the outputs.
The LTC3527-1 provides an instant off feature which
discharges V
OUT1
or V
OUT2
when their respective SHDN
pins go low.
A frequency select function allows for 1.2MHz switching
(FSEL = Low) or 2.2MHz switching (FSEL = High).
Low Voltage Start-Up
The LTC3527/LTC3527-1 include an independent start-up
oscillator designed to start up at an input voltage of 0.7V
(typical). The two converters can be started together or
in either sequence of boost 1 and boost 2 with appropri-
ate control of SHDN1 and SHDN2. Soft-start and inrush
current limiting are provided to each converter indepen-
dently during start-up, as well as during normal mode.
When V
IN
, V
OUT1
, or V
OUT2
exceeds 1.4V (typical), the IC
enters normal operating mode. Once the higher of V
OUT1
or V
OUT2
exceeds V
IN
by 0.24V, the IC powers itself from
the higher V
OUT
instead of V
IN
. At this point the internal
circuitry has no dependency on the V
IN
input voltage,
eliminating the requirement for a large input capacitor.
The input voltage can drop as low as 0.5V.
With single-cell operation, the limiting factor for the ap-
plication becomes the availability of the power source to
supply suffi cient energy to the outputs at low voltages, and
maximum duty cycle, which is clamped at 90% (typical).
Note that at low input voltages, small voltage drops due
to the higher series resistance of a depleted cell become
critical and greatly limit the power delivery capability of
the converter. A higher value, low ESR input capacitor can
help to improve this to a small degree.
Low Noise Fixed Frequency Operation
Soft-Start: The LTC3527/LTC3527-1 contain internal cir-
cuitry to provide independent soft-start operation to each
converter. The soft-start circuitry ramps the peak inductor
current from zero to its peak value of 900mA (typical)
for converter 1 or 500mA (typical) for converter 2 in ap-
proximately 0.5ms, allowing start-up into heavy loads. The
soft-start circuitry for both converters is reset in the event
of a thermal shutdown or shutdown command.
Oscillator: An internal oscillator sets the switching fre-
quency to 1.2MHz if the FSEL pin is below 0.35V, or 2.2MHz
if the FSEL pin is above 0.88V.
Shutdown: Shutdown is accomplished independently for
each converter by pulling its respective SHDN pin below
0.35V, and enabled by pulling each SHDN pin above
0.88V. Note that the SHDN pins can be driven above V
IN
or V
OUT
, as long as it is limited to less than the absolute
maximum rating.
Error Amplifier: The noninverting input of each
transconductance error amplifi er is internally connected
to the 1.20V reference. The inverting inputs are connected
(Refer to Block Diagram)
