LTC3539 LINER [Linear Technology], LTC3539 Datasheet - Page 7

• Manufacturer Part Number: LTC3539
• Description: 2A, 1MHz/2MHz Synchronous Step-Up DC/DC Converters
• Manufacturer: LINER [Linear Technology]

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 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 internal loop compensa-
tion simplifi es 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
nel MOSFET switch and P-channel MOSFET synchronous
rectifi er, the LTC3539 achieves high effi ciency over a wide
range of load current. Burst Mode operation maintains
high effi ciency 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
enters normal operating mode. Once the output voltage
exceeds the input by 0.24V, the IC powers itself from
V
no dependency on the V
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
suffi cient 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
OUT
instead of V
DS(ON)
IN
IN
or V
. At this point the internal circuitry has
and low gate charge internal N-chan-
OUT
IN
exceeds 1.4V typical, the IC
input voltage, eliminating the
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
frequency 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
or V
maximum rating.
Error Amplifi er: The error amplifi er 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
programs the output voltage via FB from 1.5V to 5.25V.
V
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 amplifi er 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
below 0.7V, in which case the current limit is cut in half.
OUT
OUT
= 1.2V • [1 + (R2/R1)].
, as long as it is limited to less than the absolute
LTC3539/LTC3539-2
OUT
to ground
OUT
falls
7
3539f
IN