LTC3539/LTC3539-2
8
35392fc
For more information www.linear.com/3539/3539-2
OPERATION
Zero Current Comparator: The zero current comparator
monitors the inductor current to the output and shuts off
the synchronous rectifier once this current reduces to ap
-
proximately 40mA. This prevents the inductor current from
reversing in polarity, improving efficiency at light loads.
Synchronous Rectifier: The P-channel MOSFET synchro
-
nous rectifier is disabled when V
OUT
is much less than V
IN
.
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
SW
(capacitance on SW pin) is low energy, but can
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 rectifier. This allows V
OUT
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
OUT
. The output
disconnect feature also allows V
OUT
to be pulled high,
without any reverse current into a battery on V
IN
.
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
enter Burst Mode operation at light load current, then
return to fixed frequency PWM mode when the load in
-
creases. Refer
to the typical performance characteristics
to see the output load Burst Mode threshold vs V
IN
. The
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
fixed frequency, using the same error amplifier 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
OUT
. When the output voltage
droops slightly, switching resumes. This maximizes ef-
ficiency at
very light loads by minimizing switching and
qu
iescent current losses. Burst Mode output voltage
ripple, which is application sensitive and typically 1% to
2% peak-to-peak if data sheet recommendations are fol
-
lowed, 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
OUT
is at least 0.24V greater
than V
IN
.
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
IN
or V
OUT
by 0.5V, the MODE pin will sink additional current.
Due to the high inductor current slew rate in 5V output
applications, the LTC3539-2 may not enter Burst Mode
operation for input voltages less than 1.8V. For 5V applica
-
tions requiring
Burst Mode operation, the 1MHz LTC3539
is recommended. Refer to the Typical Performance
Characteristics curves for the Burst Mode thresholds for
different input and output voltages.
