LTC3528B Linear Technology Corporation, LTC3528B Datasheet - Page 9

• Manufacturer Part Number: LTC3528B
• Description: 1MHz Synchronous Step-Up DC/DC Converters
• Manufacturer: Linear Technology Corporation

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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 20mA. This
prevents the inductor current from reversing in polarity,
improving effi ciency at light loads.
Synchronous Rectifi er
To control inrush current and to prevent the inductor
current from running away when V
P- channel MOSFET synchronous rectifi er is only enabled
when V
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
on SW pin) is low energy, but can cause EMI radiation.
Output Disconnect
The LTC3528/LTC3528B is designed to allow true output
disconnect by eliminating body diode conduction of the
internal P-channel MOSFET rectifi er. This allows for V
to go to zero volts during shutdown, drawing no current
from the input source. It also enables inrush current limiting
at turn-on, minimizing surge currents seen by the input
supply. Note that to obtain the advantages of output discon-
nect, a Schottky diode cannot be connected between SW
and V
to be forced above the programmed regulation voltage,
without any reverse current into a battery on V
Thermal Shutdown
If the die temperature exceeds 160°C, the LTC3528/
LTC3528B will enter thermal shutdown. All switches will be
turned off and the soft-start capacitor will be discharged.
The device will be enabled again when the die temperature
drops by approximately 15°C.
OPERATION
OUT
OUT
. The output disconnect feature also allows V
> (V
IN
+ 0.24V).
(Refer to Block Diagram)
OUT
is close to V
SW
(capacitance
IN
.
IN
, the
OUT
OUT
Burst Mode OPERATION
The LTC3528 will automatically enter Burst Mode operation
at light load current and return to fi xed frequency PWM
mode when the load increases. Refer to the Typical Perfor-
mance Characteristics to see the output load Burst Mode
threshold vs V
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 LTC3528 continues switch-
ing at a fi xed frequency of 1MHz, using the same error
amplifi er and loop compensation for peak current mode
control. This control method minimizes output transients
when switching between modes. In Burst Mode opera-
tion, energy is delivered to the output until it reaches the
nominal regulated value, then the LTC3528 transitions to
sleep mode where the outputs are off and the LTC3528
consumes only 12μA of quiescent current from V
the output voltage has drooped slightly, switching resumes
again. This maximizes effi ciency at very light loads by
minimizing switching and quiescent current losses. Burst
Mode output ripple, which is typically 1% peak-to-peak,
can be reduced by using more output capacitance (10μF
or greater).
As the load current increases, the LTC3528 automatically
leaves Burst Mode operation. Note that larger output ca-
pacitor values may cause this transition to occur at lighter
loads. The regulator will also leave Burst Mode operation if
a load transient occurs which causes the inductor current
to repeatedly reach current limit. Once the LTC3528 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 and
until soft-start is done and V
than V
The LTC3528B features continuous PWM operation at
1MHz. At very light loads, the LTC3528B will exhibit
pulse-skip operation.
IN
.
IN
. The load at which Burst Mode operation
LTC3528/LTC3528B
OUT
is at least 0.24V greater
OUT
. Once
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