LTC3527-1_15 Datasheet, PDF (11/22 Pages) Linear Technology – Dual 800mA/400mA, 1.2MHz/2.2MHz Synchronous Step-Up DC/DC Converters

LTC3527/LTC3527-1
OPERATION
to FB1 for converter 1 and FB2 for converter 2. Clamps
limit the minimum and maximum error amp output volt-
ages for improved large-signal transient response. Power
converter control loop compensation is provided internally.
An external resistive voltage divider from V_OUT1 (V_OUT2)
to ground programs the respective output voltage via FB1
(FB2) from 1.6V to 5.25V.
V_OUT1
=
1
.20V
•
⎡⎣⎢₁₊
R1⎤
R2 _⎦⎥
V_OUT2
=
1.20V
•
⎡⎣⎢₁₊
R3
R4
⎤
⎦⎥
(See Block Diagram)
Current Sensing: Lossless current sensing converts the
peak current signal of each N-channel MOSFET switch
into a voltage which is summed with its corresponding
internal slope compensation. The summed signals are
compared to their respective error amplifier outputs to
provide individual peak current control commands for the
PWM of each converter.
Current Limit: The current limit comparators shut off
the N-channel MOSFET switches once their threshold
is reached. Each current limit comparator delay time to
output is typically 60ns. Peak switch current is limited
to approximately 900mA for converter 1 and 500mA for
converter 2, independent of input or output voltage. If
V_OUT1 or V_OUT2 falls below 1V, its respective current limit
is cut in half.
Zero Current Comparator: The zero current comparators
monitor the inductor current to the outputs and shut off
the synchronous rectifiers when the current reduces to ap-
proximately 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 currents from running away when
V_OUT1 or V_OUT2 is close to V_IN, the P-channel MOSFET
synchronous rectifiers are only enabled when their respec-
tive V_OUT > (V_IN + 0.24V).
Anti-Ringing Control: The anti-ringing control connects a
resistor across the inductor to prevent high frequency ring-
ing on the SW1 (SW2) pins during discontinuous current
mode operation. Although the ringing of the resonant circuit
formed by the inductors and C_SW (capacitance on SW1 or
SW2 pins) is low energy, it can cause EMI radiation.
Output Disconnect: The LTC3527/LTC3527-1 are designed
to allow true output disconnect by eliminating body diode
conduction of the internal P-channel MOSFET rectifiers.
This allows V_OUT1 and V_OUT2 to go to zero volts during
shutdown, drawing no current from the input source. It
also allows for inrush current limiting at turn-on, minimiz-
ing surge currents seen by the input supply. Note that to
obtain the advantages of output disconnect, there must
not be external Schottky diodes connected between the
SW1 (SW2) pins and V_OUT1 (V_OUT2). The output discon-
nect feature also allows V_OUT1 or V_OUT2 to be pulled high,
without any reverse current into a battery on V_IN.
Thermal Shutdown: If the die temperature exceeds 160°C,
the device will go into thermal shutdown. All switches
will be turned off and the soft-start capacitors will be
discharged. The device will be enabled again when the
die temperature drops by about 15°C.
Burst Mode Operation
To realize the efficiency benefits of Burst Mode operation,
both V_OUT1 and V_OUT2 must be under a light load current
condition, if they are both enabled. If one converter is shut
down, then Burst Mode operation is enabled on the other
converter. With the MODE pin low, the LTC3527/LTC3527-1
will automatically enter Burst Mode operation 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 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 is operation entered.
In Burst Mode operation, the LTC3527/LTC3527-1 still
switch at a fixed frequency of 1.2MHz (FSEL = 0) or 2.2MHz
(FSEL = 1), using the same error amplifier and loop compen-
sation for peak current mode control. This control method
eliminates the output transient when switching between
modes. In Burst Mode operation, energy is delivered to the
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