LTC3526L-2/LTC3526LB-2
9
3526lb2fa
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-ring circuit connects a resistor across the in-
ductor 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 LTC3526L-2/LTC3526LB-2 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 cur-
rent 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 SW and V
OUT
. The output disconnect
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 LTC3526L-2/
LTC3526LB-2 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 LTC3526L-2 will enter Burst Mode operation at light
load current 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 LTC3526L-2 still switches at
a fixed frequency of 2MHz, 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 LTC3526L-2 transi-
tions to sleep mode where the outputs are off and the
LTC3526L-2 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) con-
nected between V
OUT
and FB.
As the load current increases, the LTC3526L-2 will au-
tomatically leave Burst Mode operation. Note that larger
output capacitor values may cause this transition to oc-
cur at lighter loads. Once the LTC3526L-2 has left Burst
Mode operation and returned to normal operation, it will
remain there until the output load is reduced below the
burst threshold current.
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 LTC3526LB-2 features continuous PWM operation at
2MHz. At very light loads, the LTC3526LB-2 will exhibit
pulse-skipping operation.
(Refer to Block Diagram)
operaTion
