LTC3537
12
3537fd
OPERATION
Boost Current Limit
The current limit comparator shuts off the N-channel
MOSFET switch once its threshold is reached. The cur-
rent limit comparator delay to output is typically 40ns.
Peak switch current is limited to approximately 750mA,
independent of input or output voltage, unless V
OUTB
falls
below 0.8V, in which case the current limit is cut in half.
Boost 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 reduces to approximately 30mA. This
prevents the inductor current from reversing in polarity,
improving effi ciency at light loads.
Boost Synchronous Rectifi er
To control inrush current and to prevent the inductor cur-
rent from running away when V
OUTB
is close to V
INB
, the
P-channel MOSFET synchronous rectifi er is only enabled
when V
OUTB
> (V
INB
+ 0.24V).
Boost 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. Although
the ringing of the resonant circuit formed by L and C
SW
(capacitance on SW pin) is low energy, it can cause EMI
radiation.
Boost Output Disconnect
The LTC3537 is designed to allow true output disconnect
by eliminating body diode conduction of the internal P-
channel MOSFET synchronous rectifi er. This allows V
OUTB
to go to zero volts during shutdown, drawing no current
from the input source. It also allows inrush current limit-
ing at turn-on, minimizing surge currents seen by the
input supply. Note that to obtain the advantages of output
disconnect, there cannot be an external Schottky diode
connected between the SW pin and V
OUTB
. The output
disconnect feature also allows V
OUTB
to be pulled high,
above the nominal regulation voltage, without any reverse
current into the power source connected to V
INB
.
Thermal Overload Protection
If the die temperature exceeds 160°C typical, the LTC3537
boost converter will shut down. All switches will be off
and the soft-start capacitor will be discharged. The boost
converter will be enabled when the die temperature drops
by approximately 15°C.
BOOST BURST MODE OPERATION
When enabled (MODE pin high), the LTC3537 will auto-
matically enter Burst Mode operation at light load current
and return to fi xed frequency PWM mode when the load
increases. Refer to the Typical Performance Characteristics
to see the Burst Mode Threshold Current vs V
INB
. 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 LTC3537 still switches at a
fi xed frequency of 2.2MHz, 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, en-
ergy is delivered to the output until it reaches the nominal
voltage regulation value, then the LTC3537 transitions to
sleep mode where the outputs are off and the LTC3537
consumes only 30µA of quiescent current from V
OUTB
including the current required to keep the LDO enabled.
When the output voltage droops slightly, switching re-
sumes. This maximizes effi ciency at very light loads by
minimizing switching and quiescent losses. Burst Mode
output voltage ripple, which is typically 1% peak-to-peak,
can be reduced by using more output capacitance (10µF
or greater), or with a small capacitor (10pF to 50pF) con-
nected between V
OUTB
and FBB.
As the load current increases, the LTC3537 will automati-
cally leave Burst Mode operation. Note that larger output
capacitor values may cause this transition to occur at lighter
loads. Once the LTC3537 has left Burst Mode operation and
returned to normal operation, it will remain there until the
output load is reduced below the burst threshold.
