LTC3527-1_15 Datasheet, PDF (10/22 Pages) Linear Technology – Dual 800mA/400mA, 1.2MHz/2.2MHz Synchronous Step-Up DC/DC Converters

LTC3527/LTC3527-1
OPERATION (Refer to Block Diagram)
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 fixed
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 filtering. Internal
soft-start and loop compensation simplifies 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
rectifiers, the LTC3527/LTC3527-1 achieve high efficiency
over a wide range of load current. With the MODE pin low,
automatic Burst Mode operation maintains high efficiency
at very light loads, reducing the quiescent current to just
12μA. If MODE is high, fixed 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 sufficient 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 amplifier is internally connected
to the 1.20V reference. The inverting inputs are connected
35271fc
10