LTC3529 Linear Dimensions Semiconductor, LTC3529 Datasheet - Page 7

• Manufacturer Part Number: LTC3529
• Description: 1.5MHz Step-Up dc
• Manufacturer: Linear Dimensions Semiconductor

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OPERATION
Oscillator
An internal oscillator sets the switching frequency to
1.5MHz.
Shutdown
The LTC3529 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
is limited to less than the absolute maximum rating.
Error Amplifi er
The error amplifi er is a transconductance amplifi er with
an internal compensation network. Internal clamps limit
the minimum and maximum error amplifi er output voltage
to improve the large-signal transient response.
Current Sensing
Lossless current sensing converts the peak current sig-
nal 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 amplifi er output
to provide a peak current control command for the PWM.
Peak switch current is limited to approximately 2A inde-
pendent of input or output voltage.
Current Limit
The current limit comparator shuts off the N-channel MOSFET
switch when the current limit threshold is reached. The current
limit comparator delay time to output is typically 40ns.
Fault Detection
To prevent the device from providing power to a shorted
output, the switch current is monitored to detect an
overcurrent condition. In the event that the switch current
reaches the current limit for longer than 22ms, the fault
fl ag is asserted (FAULT pulls low) and the device is shut
down. If the auto-restart option is enabled (RST high), the
device will automatically attempt to restart every 22ms
until the short is removed. If auto-restart is disabled (RST
low), the IC will remain shut down until being manually
restarted by toggling SHDN or cycling the input voltage.
A soft-start sequence is initiated when the device restarts.
If output short circuits are common in the application,
IN
or V
OUT
, as long as it
latchoff mode is highly recommended for maximum level
of robustness.
Note: When V
tion, it is possible for the output to momentarily exceed
the maximum output voltage rating. In cases where
repeated shorts are expected, V
by the addition of a 5.6V Zener clamp from V
Alternatively, C
Zero-Current Comparator
The zero-current comparator monitors the inductor cur-
rent to the output and shuts off the synchronous rectifi er
when this current falls below approximately 20mA. This
prevents the inductor current from reversing in polarity,
thereby improving effi ciency at light loads.
Anti-Ringing Control
The anti-ringing circuit connects a resistor across the
inductor to damp the ringing on SW in discontinuous
conduction mode. The ringing of the resonant circuit
formed by L and C
energy but can cause EMI radiation.
Output Disconnect
The LTC3529 provides true output disconnect by eliminating
body diode conduction of the internal P-channel MOSFET
rectifi er. This allows V
down, drawing no current from the input source. It also
provides inrush current limiting at turn-on, minimizing
surge currents seen by the input supply.
Thermal Shutdown
If the die temperature reaches approximately 160°C,
the device enters thermal shutdown, the fault fl ag is
asserted (FAULT pulls low) and all switches are turned off.
The device is enabled and a soft-start sequence is initiated
when the die temperature drops by approximately 10°C.
PCB Layout
Due to the high frequency operation of the LTC3529, board
layout is extremely critical to minimize transients caused by
stray inductance. Keep the output fi lter capacitor as close
as possible to the V
ceramic capacitors tied to a good ground plane.
OUT
OUT
is released from a short-circuit condi-
SW
can be increased to 47μF or greater.
OUT
OUT
(capacitance on the SW pin) is low
pin and use very low ESR/ESL
to go to zero volts during shut-
OUT
should be protected
LTC3529
OUT
to GND.
3529fa
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