LTC3442
U
OPERATIO
V
V
D
IN
OUT
8
Boost Region (VIN < VOUT
)
9
SwitchAisalwaysonandswitchBisalwaysoffduringthis
mode. When the internal control voltage, VCI, 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 VCI is above V4.
V
dI
dt
OUT
L
+
A
≈ –
–
900mA
0mA
4
6
L
SW1
SW2
B
C
3442 F04
T2
5
GND
BURST MODE OPERATION
Figure 4. Inductor Discharge Cycle During 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 VIN. 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. BurstModeoperationripplecanbereduced
slightlybyusingmoreoutputcapacitance(47µForgreater).
Another method of reducing Burst Mode operation ripple
istoplaceasmallfeed-forwardcapacitoracrosstheupper
resistor in the VOUT feedback divider network (as in Type
III compensation).
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:
n •ILOAD
EFFICIENCY ≅
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:
35µA +ILOAD
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
enterBurstModeoperationatlightloadandreturntofixed
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:
0.2 • V
IN
IOUT(MAX)BURST
≈
A
VOUT + V
IN
Note that the peak efficiency during Burst Mode operation
is less than the peak efficiency during fixed frequency
V
V
IN
OUT
9
8
V
L
dI
dt
IN
A
D
≈
17.6
Enter Burst Mode: I =
RBURST
+
–
900mA
0mA
4
6
L
SW1
SW2
22.4
B
C
Leave Burst Mode: I =
3442 F03
T1
RBURST
5
GND
where RBURST is in kΩ and IBURST is the load transition
Figure 3. Inductor Charge Cycle During Burst Mode Operation
3442f
10
Linear Systems [ Linear Systems ]
