ltc3527eud-trpbf Linear Technology Corporation, ltc3527eud-trpbf Datasheet - Page 11

• Manufacturer Part Number: ltc3527eud-trpbf
• Description: Dual 800ma/400ma, 1.2mhz/2.2mhz Synchronous Step-up Dc/dc Converters
• Manufacturer: Linear Technology Corporation

OPERATION
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
to ground programs the respective output voltage via FB1
(FB2) from 1.6V to 5.25V.
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 amplifi er 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
is cut in half.
Zero Current Comparator: The zero current comparators
monitor the inductor current to the outputs and shut off
the synchronous rectifi ers when the current reduces to ap-
proximately 30mA. This prevents the inductor current from
reversing in polarity, improving effi ciency at light loads.
Synchronous Rectifi er: To control inrush current and
to prevent the inductor currents from running away when
V
synchronous rectifi ers are only enabled when their respec-
tive V
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
SW2 pins) is low energy, it can cause EMI radiation.
OUT1
OUT1
V
V
OUT
OUT
OUT
or V
or V
1
2
> (V
=
=
OUT2
OUT2
1 20
1 20
.
.
IN
falls below 1V, its respective current limit
+ 0.24V).
is close to V
V
V
•
•
⎡
⎢
⎣
⎡
⎢
⎣ ⎣
1
1
+
+
R
R
R
R
2
1
3
4
⎤
⎥
⎦
⎤
⎥ (
⎦
IN
SW
, the P-channel MOSFET
(capacitance on SW1 or
See Block Diagram
OUT1
(V
OUT2
)
)
Output Disconnect: The LTC3527/LTC3527-1 are
designed to allow true output disconnect by eliminating
body diode conduction of the internal P-channel MOSFET
rectifi ers. This allows V
during shutdown, drawing no current from the input source.
It also allows for inrush current limiting at turn-on, mini-
mizing 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
nect feature also allows V
without any reverse current into a battery on V
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 effi ciency benefi ts of Burst Mode operation,
both V
condition, if they are both enabled. If one channel is shut
down, then Burst Mode operation is enabled on the other
channel. With the MODE pin low, the LTC3527/LTC3527-1
will automatically enter Burst Mode operation at light load
and return to fi xed frequency PWM mode when the load
increases. Refer to the Typical Performance Characteristics
to see the Output Load Burst Mode Threshold Current vs
V
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
switches at a fi xed frequency of 1.2MHz (FSEL = 0) or
2.2MHz (FSEL = 1), using the same error amplifi er and
loop compensation 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 output until it reaches the
nominal regulation value, then the LTC3527/LTC3527-1
transitions to sleep mode where the outputs are off and
the LTC3527/LTC3527-1 consumes only 12µA of quies-
cent current from the higher of V
IN
. The load current at which Burst Mode operation is
OUT1
and V
LTC3527/LTC3527-1
OUT2
must be under a light load current
OUT1
OUT1
OUT1
and V
(V
or V
OUT2
OUT2
OUT2
OUT1
). The output discon-
to go to zero volts
to be pulled high,
or V
OUT2
IN
.
11
. When
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