LTC3527 Linear Technology Corporation, LTC3527 Datasheet - Page 10

• Manufacturer Part Number: LTC3527
• Description: 1.2MHz/2.2MHz Synchronous Step-Up DC/DC Converters
• Manufacturer: Linear Technology Corporation

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OPERATION
LTC3527/LTC3527-1
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
pin must be wired to the higher of the two supplies and
each V
capacitors are recommended on all V
With low R
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 out-
puts. 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 ⎯ S ⎯ H ⎯ D ⎯ N pins to provide
sequencing of the outputs.
The LTC3527-1 provides an instant off feature which
discharges V
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 appropriate
control of ⎯ S ⎯ H ⎯ D ⎯ N ⎯ 1 and ⎯ S ⎯ H ⎯ D ⎯ N ⎯ 2 . Soft-start and inrush cur-
10
OUT
DS(ON)
must be higher than the highest V
OUT1
and low gate charge internal N-channel
or V
(Refer to Block Diagram)
OUT2
when their respective ⎯ S ⎯ H ⎯ D ⎯ N
IN
pins.
IN
. Bypass
IN
rent limiting are provided to each converter independently
during start-up, as well as during normal mode.
When V
enters normal operating mode. Once the higher of V
or V
the higher V
circuitry has no dependency on the V
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 channels 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 ⎯ S ⎯ H ⎯ D ⎯ N pin below
0.35V, and enabled by pulling each ⎯ S ⎯ H ⎯ D ⎯ N pin above
0.88V. Note that the ⎯ S ⎯ H ⎯ D ⎯ N pins can be driven above V
or V
maximum rating.
Error Amplifier: The non-inverting input of each
transconductance error amplifi er is internally connected
to the 1.20V reference. The inverting inputs are connected
to FB1 for converter 1 and FB2 for converter 2. Clamps
OUT2
OUT
, as long as it is limited to less than the absolute
IN
exceeds V
, V
OUT1
OUT
instead of V
, or V
IN
by 0.24V, the IC powers itself from
OUT2
exceeds 1.4V (typical), the IC
IN
. At this point the internal
IN
input voltage,
OUT1
35271f
IN