LTC3442 Datasheet, PDF (10/20 Pages) Linear Integrated Systems – Micropower Synchronous Buck-Boost DC/DC Converter with Automatic Burst Mode Operation

LTC3442
U
OPERATIO
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 pro-
vide 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 feed-forward 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
V_IN
V_OUT
9
8
A
dI
dt
≈
V_IN
L
D
+
–
4
6
L
SW1
SW2
B
C
900mA
0mA
T1
3442 F03
5
GND
Figure 3. Inductor Charge Cycle During Burst Mode Operation
10
V_IN
V_OUT
9
8
A
dI
dt
≈
–
V_OUT
L
D
–
+
4
6
SW1
L
SW2
B
C
900mA
0mA
5
GND
T2
3442 F04
Figure 4. Inductor Discharge Cycle During Burst Mode Operation
because the part enters full-time 4-switch mode (when
servicing the output) with discontinuous inductor current
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 re-
quired to maintain the Buck/Boost function. The efficiency
below 1mA becomes dominated primarily by the quies-
cent current. The Burst Mode operation efficiency is given
by:
EFFICIENCY ≅ ^{n • ILOAD}
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
controlled 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
3442f