LTC3526/LTC3526B
OPERATION
(Refer to Block Diagram)
Zero Current Comparator
The zero current comparator monitors the inductor cur-
rent to the output and shuts off the synchronous rectifier
when this current reduces to approximately 30mA. This
prevents the inductor current from reversing in polarity,
improving efficiency at light loads.
Synchronous Rectifier
To control inrush current and to prevent the inductor
current from running away when V
OUT
is close to V
IN
, the
P-channel MOSFET synchronous rectifier is only enabled
when V
OUT
> (V
IN
+ 0.24V).
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.
Output Disconnect
The LTC3526/LTC3526B are designed to allow true output
disconnect by eliminating body diode conduction of the
internal P-channel MOSFET rectifier. This allows for V
OUT
to go to zero volts during shutdown, drawing no current
from the input source. It also allows for 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 dis-
connect feature also allows V
OUT
to be pulled high, without
any reverse current into a battery connected to V
IN
.
Thermal Shutdown
If the die temperature exceeds 160°C, the LTC3526/
LTC3526B will go into thermal shutdown. All switches
will be off and the soft-start capacitor will be discharged.
The device will be enabled again when the die temperature
drops by about 15°C.
Burst Mode OPERATION
The LTC3526 will automatically enter Burst Mode opera-
tion at light load and return to fixed frequency PWM mode
when the load increases. Refer to the Typical Performance
Characteristics to see the output load Burst Mode thresh-
old current 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 LTC3526 still switches at a
fixed frequency of 1MHz, 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 opera-
tion, energy is delivered to the output until it reaches the
nominal regulation value, then the LTC3526 transitions to
sleep mode where the outputs are off and the LTC3526
consumes only 9μA of quiescent current from V
OUT
. When
the output voltage droops slightly, switching resumes.
This maximizes efficiency 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) connected between
V
OUT
and FB.
As the load current increases, the LTC3526 will automati-
cally leave Burst Mode operation. Note that larger output
capacitor values may cause this transition to occur at lighter
loads. Once the LTC3526 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 soft-
start and until V
OUT
is at least 0.24V greater than V
IN
.
The LTC3526B features continuous PWM operation at
1MHz. At very light loads, the LTC3526B will exhibit
pulse-skip operation.
3526bfb
9
LINER [ LINEAR TECHNOLOGY ]
