Datasheet

LTC3442
10
3442fb
For more information www.linear.com/LTC3442
OPERATION
Boost Region (V
IN
< V
OUT
)
Switch A is always on and switch B is always off during
this mode. When the internal control voltage, V
CI
, is above
voltage V3, switch pair CD will alternately switch to provide
a boosted output voltage. This operation is typical to a
synchronous boost regulator. The maximum duty cycle
of the converter is limited to 88% typical and is reached
when
V
CI
is above V4.
BURST MODE OPERATION
Burst Mode operation occurs when the IC delivers energy
to the output until it is regulated and then goes into a sleep
mode where the outputs are off and the IC is consuming
only 35µA of quiescent current from V
IN
. In this mode the
output ripple has a variable frequency component that
depends upon load current, and will typically
be about
2% peak-to-peak. Burst Mode operation ripple can be
reduced slightly by using more output capacitance (47µF
or greater). Another method of reducing Burst Mode
operation ripple is to place a small feedforward capacitor
across the upper resistor in the V
OUT
feedback divider
network (as in TypeIII compensation).
During the period where the device is delivering energy to
the output, the peak switch
current will be equal to 900mA
typical and the inductor current will terminate at zero
current for each cycle. In this mode the typical maximum
average output current is given by:
I
OUT(MAX)BURST
≈
0.2 • V
IN
V
OUT
+ V
IN
A
Note that the peak efficiency during Burst Mode operation
is less than the peak efficiency during fixed frequency
9
V
IN
A
4
SW1
5
GND
6
SW2
L
+ –
8
V
OUT
D
C
900mA
I
INDUCTOR
0mA
3442 F03
T1
B
dI
dt
V
IN
L
Figure 3. Inductor Charge Cycle During Burst Mode Operation
Figure 4. Inductor Discharge Cycle During Burst Mode Operation
9
V
IN
A
4
SW1
5
GND
6
SW2
L
– +
8
V
OUT
D
C
900mA
I
INDUCTOR
0mA
3442 F04
T2
B
dI
dt
V
OUT
L
–
because the part enters full-time 4-switch mode (when
servicing the output) with discontinuous inductor cur-
rent as illustrated in Figures 3 and 4. During Burst Mode
operation, the control loop is nonlinear and cannot utilize
the control voltage from the error amp to determine the
control mode, therefore full-time 4-switch mode is required
to maintain the buck/boost function. The efficiency below
1mA becomes dominated primarily by
the quiescent cur-
rent. The Burst Mode operation efficiency is given by:
EFFICIENCY ≅
n • I
LOAD
35µA +I
LOAD
where n is typically 82% during Burst Mode operation.
Automatic Burst Mode Operation Control
Burst Mode operation can be automatic or manually con-
trolled with a single pin. In automatic mode, the IC will
enter Burst Mode operation at light load and return to fixed
frequency operation at heavier loads. The load current at
which the mode transition occurs is programmed using
a single external resistor from the
BURST pin to ground,
according to the following equations:
Enter Burst Mode: I =
17.6
R
BURST
Leave Burst Mode: I =
22.4
R
BURST
where R
BURST
is in kΩ and I
BURST
is the load transition
current in Amps. Do not use values of R
BURST
greater
than 250k.