LTC3539-2 Linear Technology, LTC3539-2 Datasheet - Page 8

• Manufacturer Part Number: LTC3539-2
• Description: 1MHz/2MHz Synchronous Step-Up DC/DC Converters
• Manufacturer: Linear Technology

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LTC3539/LTC3539-2
OPERATION
Zero Current Comparator: The zero current comparator
monitors the inductor current to the output and shuts off
the synchronous rectifi er once this current reduces to ap-
proximately 40mA. This prevents the inductor current from
reversing in polarity, improving effi ciency at light loads.
Synchronous Rectifi er: The P-channel MOSFET synchro-
nous rectifi er is disabled when V
This is to control inrush current and to prevent inductor
current from running away
Anti-ringing Control: The anti-ringing control connects
a resistor across the inductor to prevent high frequency
ringing on the SW pin during discontinuous current mode
operation. The ringing of the resonant circuit formed by L
and C
cause EMI radiation.
Output Disconnect: The LTC3539 is designed to allow true
output disconnect by eliminating body diode conduction of
the internal P-channel MOSFET rectifi er. This allows V
to go to zero volts during shutdown, drawing no 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 an external Schottky
diode connected between the SW pin and V
disconnect feature also allows V
without any reverse current into a battery on V
Thermal Shutdown: If the die temperature exceeds 160°C,
the device will go into thermal shutdown. All switches will
be turned off and the internal soft-start capacitor will be
discharged. The device will be enabled again when the die
temperature drops by about 15°C.
Burst Mode Operation
When Burst Mode operation is enabled by bringing the
MODE pin above 0.88V, the LTC3539 will automatically
8
SW
(capacitance on SW pin) is low energy, but can
OUT
OUT
is much less than V
to be pulled high,
OUT
. The output
IN
.
OUT
IN
.
enter Burst Mode operation at light load current, then
return to fi xed frequency PWM mode when the load in-
creases. Refer to the typical performance characteristics
to see the output load Burst Mode threshold vs V
load current at which Burst Mode operation is entered can
be changed by adjusting the inductor value. Raising the
inductor value will lower the load current at which Burst
Mode operation is entered.
In Burst Mode operation, the LTC3539 still switches at a
fi xed frequency, using the same error amplifi er and loop
compensation for peak current mode control. This control
method eliminates any output transient when switching
between modes. In Burst Mode operation, energy is deliv-
ered to the output until it reaches the nominal regulation
value, then the LTC3539 transitions to Sleep mode where
the outputs are off and the LTC3539 consumes only 10μA
of quiescent current from V
droops slightly, switching resumes. This maximizes ef-
fi ciency at very light loads by minimizing switching and
quiescent current losses. Burst Mode output voltage ripple,
which is typically 1% peak to peak, can be reduced by
using more output capacitance (47μF or greater).
As the load increases, the LTC3539 will automatically leave
Burst Mode operation. Once the LTC3539 has left Burst
Mode operation and returned to normal operation, it will
remain there until the output load is reduced below the
burst threshold.
Burst Mode operation is inhibited during start-up until
soft-start is complete and V
than V
When the MODE pin is below 0.3V, the LTC3539 features
continuous PWM operation. In this mode, at very light
loads, the LTC3539 will exhibit pulse-skip operation. If the
MODE pin voltage exceeds the greater of V
0.5V, the MODE pin will sink additional current.
IN
.
OUT
OUT
. When the output voltage
is at least 0.24V greater
IN
or V
IN
OUT
. The
3539f
by