LTC3526L_15 Datasheet, PDF (9/16 Pages) Linear Technology – 550mA 1MHz Synchronous Step-Up DC/DC Converters

LTC3526L/LTC3526LB
OPERATION (Refer to Block Diagram)
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/LTC3526LB 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 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/LT-
C3526LB 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 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 Character-
istics to see the output load Burst Mode threshold 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 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 LTC3526L transitions
to sleep mode where the outputs are off and the LTC3526L
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 LTC3526L will auto-
matically leave Burst Mode operation. Note that larger
output capacitor values may cause this transition to
occur at lighter loads. Once the LTC3526L 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 features continuous PWM operation at
1MHz. At very light loads, the LTC3526LB will exhibit
pulse-skip operation.
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