LTC3535_15 Datasheet, PDF (11/16 Pages) Linear Technology – Dual Channel 550mA 1MHz Synchronous Step-Up DC/DC Converter

LTC3535
OPERATION ^{(Refer to Block Diagram)}
Burst Mode OPERATION
Each channel of the LTC3535 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 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 LTC3535 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 operation,
energy is delivered to the output until it reaches the
nominal regulation value, then the LTC3535 transitions
to sleep mode where the outputs are off and the LTC3535
consumes only 9µA of quiescent current from V_OUT for
each channel. 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 LTC3535 will automati-
cally leave Burst Mode operation. Note that larger output
capacitor values may cause this transition to occur at
lighter loads. Once the LTC3535 has left Burst Mode op-
eration 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.
Note that each channel can enter or leave Burst Mode
operation independent of the other channel.
APPLICATIONS INFORMATION
V_IN > V_OUT OPERATION
The LTC3535 will maintain voltage regulation even when
the input voltage is above the desired output voltage. Note
that the efficiency is much lower in this mode, and the
maximum output current capability will be less. Refer to
the Typical Performance Characteristics.
SHORT-CIRCUIT PROTECTION
The LTC3535 output disconnect feature allows output
short circuit while maintaining a maximum internally set
current limit. To reduce power dissipation under short-
circuit conditions, the peak switch current limit is reduced
to 400mA (typical per channel).
SCHOTTKY DIODE
Although not recommended, adding a Schottky diode from
SW to V_OUT will improve efficiency by about 2%. Note
that this defeats the output disconnect and short-circuit
protection features.
PCB LAYOUT GUIDELINES
The high speed operation of the LTC3535 demands careful
attention to board layout. A careless layout will result in
reduced performance. Figure 2 shows the recommended
component placement. A large ground pin copper area
will help to lower the die temperature. A multilayer board
with a separate ground plane is ideal, but not absolutely
necessary.
SHDN
V_OUT1
V_IN1
GND
GND
V_OUT2
V_IN2
SHDN
Figure 2. Recommended Component Placement
3535fa
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