LTC3427_15 Datasheet, PDF (6/12 Pages) Linear Technology – 500mA, 1.25MHz Synchronous Step-Up DC/DC Converter in 2mm 2mm DFN Package

LTC3427
U
OPERATIO (Refer to Block Diagram)
LOW NOISE FIXED FREQUENCY OPERATION
Shutdown
The LTC3427 is shut down by pulling the SHDN pin below
0.35V, and activated by pulling the SHDN pin above 1V.
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.
Soft-Start
The LTC3427 provides soft-start by ramping the peak
inductor current from zero to its peak value of 500mA. The
soft-start time is typically 2ms. A soft-start cycle is re-
initiated in the event of a commanded shutdown or a
thermal shutdown.
Oscillator
The frequency of operation is set by an internal oscillator
to 1.25MHz for the LTC3427.
Error Amplifier
The error amplifier is a transconductance type with its
positive input internally connected to the 1.24V reference
and the negative input connected to FB. Internal clamps
limit the minimum and maximum error amplifier output
voltage for improved large-signal transient response.
Power converter control loop compensation is provided
internally by the error amplifier. A voltage divider from
V_OUT to ground programs the output voltage via FB from
1.8V to 5.25V.
V_OUT
=
1.24V
•
⎛
⎝⎜
1+
R1⎞
R2 ⎠⎟
The error amplifier also provides a soft-start feature
internal to the device.
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. Peak
switch current is limited to 500mA minimum, independent
6
of input or output voltage. The current signal is blanked for
approximately 25ns to enhance noise rejection.
Current Limit
The current limit circuitry shuts off the internal N-channel
MOSFET switch when the current limit threshold is reached.
The current limit comparator delay to output is typically
40ns.
Zero Current Comparator
The zero current comparator monitors the inductor cur-
rent to the output and shuts off the synchronous rectifier
once this current reduces to approximately 20mA.
Anti-Ringing Control
The anti-ringing control connects a resistor across the
inductor to damp the ringing on the SW pin in discontinu-
ous conduction mode. The LC_SW ringing (L = Inductor,
C_SW = capacitance on the SW pin) is low energy, but can
cause EMI radiation.
Output Disconnect and Inrush Limiting
The LTC3427 provides true output disconnect by elimi-
nating body diode conduction of the internal P-channel
MOSFET rectifier. This allows V_OUT to go to zero volts
during shutdown without drawing any current from the
input source. It also provides inrush current limiting at
turn-on, minimizing surge currents seen by the input
supply. Note that to obtain the advantages of output
disconnect, there must not be any external Schottky diode
connected between SW and V_OUT.
Thermal Shutdown
If the die temperature reaches approximately 145°C, the
part will go into thermal shutdown. All switches will be
turned off. The device will be enabled and initiate a soft-
start sequence when the die temperature drops by
approximately 10°C.
Note: Due to the high frequency operation of the LTC3427,
board layout is extremely critical to minimize transients
due to stray inductance. Keep the output filter capacitor as
close as possible to the V_OUT pin and use very low ESR/
ESL ceramic capacitors tied to a good ground plane.
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