LTC3539_15 Datasheet, PDF (7/14 Pages) Linear Technology – 2A, 1MHz/2MHz Synchronous Step-Up DC/DC Converters

LTC3539/LTC3539-2
OPERATION
DETAILED DESCRIPTION
The LTC3539 is a high power synchronous boost converter
in an 8-lead 2mm × 3mm DFN package. With the ability
to start up and operate from inputs of 700mV, the 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. The switching frequency of the
LTC3539 is nominally 1MHz, while the LTC3539-2 switches
at 2MHz. Operation of the LTC3539 and LTC3539-2 is
identical in all other respects.
With its low R_DS(ON) and low gate charge internal N-channel
MOSFET switch and P-channel MOSFET synchronous
rectifier, the LTC3539 achieves high efficiency over a wide
range of load current. Burst Mode operation maintains
high efficiency at very light loads, reducing the quiescent
current to just 10µA.
Converter operation can be best understood by referring
to the Block Diagram.
Low Voltage Start-Up
The LTC3539 includes an independent start-up oscillator
designed to start up at an input voltage of 0.7V (typical).
Soft-start and inrush current limiting are provided during
start-up, as well as normal mode.
When either V_IN or V_OUT exceeds 1.4V typical, the IC
enters normal operating mode. Once the output voltage
exceeds the input by 0.24V, the IC powers itself from
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. The limiting factor for the appli-
cation becomes the ability of the power source to supply
sufficient energy to the output at the low voltages, and the
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 LTC3539/LTC3539-2 contains internal
circuitry to provide soft-start operation. The internal
soft-start circuitry ramps the peak inductor current from
zero to its peak value of 2.6A (typical) in approximately
0.5ms, allowing start-up into heavy loads. The soft-start
circuitry is reset in the event of a thermal shutdown or
shutdown command.
Oscillator: An internal oscillator sets the switching fre-
quency to 1MHz for the LTC3539, and 2MHz for the
LTC3539-2.
Shutdown: The part is shutdown by pulling the SHDN
pin below 0.3V, and activated by pulling the SHDN pin
above 0.88V. Note that SHDN 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 error amplifier is a transconductance
type. The non-inverting input is internally connected to
the 1.2V reference and the inverting input is connected
to FB. Clamps limit the minimum and maximum error
amp output voltage for improved large signal transient
response. Power converter control loop compensation is
provided internally. A voltage divider from V_OUT to ground
programs the output voltage via FB from 1.5V to 5.25V.
V_OUT = 1.2V • [1 + (R2/R1)].
Current Sensing: Lossless current sensing converts the
peak current signal of the N-channel MOSFET switch
into a voltage which 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.
Peak switch current is limited to approximately 2.6A,
independent of input or output voltage, unless V_OUT falls
below 0.7V, in which case the current limit is cut in half.
For more information www.linear.com/3539/3539-2
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