LTC3526LB Просмотр технического описания (PDF) - Linear Technology
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LTC3526LB Datasheet PDF : 16 Pages
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LTC3526L/LTC3526LB
OPERATION (Refer to Block Diagram)
The LTC3526L/LTC3526LB are 1MHz synchronous boost
converters housed in a 6-lead 2mm × 2mm DFN package.
With a guaranteed ability to start up and operate from
inputs less than 0.8V, this device features 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 internal loop compensation simplifies the
design process while minimizing the number of external
components.
With its low RDS(ON) and low gate charge internal N-chan-
nel MOSFET switch and P-channel MOSFET synchronous
rectifier, the LTC3526L achieves high efficiency over a wide
range of load currents. Burst Mode operation maintains
high efficiency at very light loads, reducing the quiescent
current to just 9μA. Operation can be best understood by
referring to the Block Diagram.
LOW VOLTAGE START-UP
The LTC3526L/LTC3526LB include an independent start-
up oscillator designed to start up at an input voltage of
0.68V (typical). Soft-start and inrush current limiting are
provided during start-up, as well as normal mode.
When either VIN or VOUT exceeds 1.3V typical, the IC
enters normal operating mode. When the output voltage
exceeds the input by 0.24V, the IC powers itself from
VOUT instead of VIN. At this point the internal circuitry has
no dependency on the VIN input voltage, eliminating the
requirement for a large input capacitor. The input voltage
can drop as low as 0.5V. The limiting factor for the ap-
plication becomes the availability of the power source to
supply sufficient energy to the output at low voltages, and
maximum duty cycle, which is clamped at 90% typical.
Note that at low input voltages, small voltage drops due
to series resistance become critical, and greatly limit the
power delivery capability of the converter.
LOW NOISE FIXED FREQUENCY OPERATION
Soft-Start
The LTC3526L/LTC3526LB contain internal circuitry to
provide soft-start operation. The soft-start circuitry slowly
8
ramps the peak inductor current from zero to its peak value
of 750mA (typical) in approximately 0.5ms, allowing start-
up into heavy loads. The soft-start circuitry is reset in the
event of a shutdown command or a thermal shutdown.
Oscillator
An internal oscillator sets the switching frequency to
1MHz.
Shutdown
Shutdown is accomplished by pulling the ⎯S⎯H⎯D⎯N pin below
0.3V and enabled by pulling the S⎯ H⎯ D⎯ N⎯ pin above 0.8V. Note
that ⎯S⎯H⎯D⎯N can be driven above VIN or VOUT, as long as it
is limited to less than the absolute maximum rating.
Error Amplifier
The positive input of the transconductance error amplifier
is internally connected to the 1.195V reference and the
negative input is connected to FB. Clamps limit the mini-
mum and maximum error amp output voltage for improved
large-signal transient response. Power converter control
loop compensation is provided internally. An external
resistive voltage divider from VOUT to ground programs
the output voltage via FB from 1.5V to 5.25V.
VOUT
=
1.195V
•
1+
R2
R1
Current Sensing
Lossless current sensing converts the peak current signal of
the N-channel MOSFET switch into a voltage that is summed
with the internal slope compensation. The summed signal
is compared to the error amplifier output to provide a peak
current control command for the PWM.
Current Limit
The current limit comparator shuts off the N-channel
MOSFET switch once its threshold is reached. The cur-
rent limit comparator delay to output is typically 60ns.
Peak switch current is limited to approximately 750mA,
independent of input or output voltage, unless VOUT falls
below 0.7V, in which case the current limit is cut in half.
3526lfb
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