LTC3533 Datasheet, PDF (9/16 Pages) Linear Technology – 2A Wide Input Voltage Synchronous Buck-Boost DC/DC Converter

LTC3533
OPERATION
D4_SW = (150ns • f) • 100 %
where f = operating frequency, Hz.
Beyond this point the “four switch,” or buck/boost region
is reached.
Buck/Boost or Four Switch (V_IN ~ V_OUT)
When the control voltage, V_C, is above voltage V2, switch
pair AD remain on for duty cycle D_{MAX_BUCK}, and switch
pair AC begins to phase in. As switch pair AC phases in,
switch pair BD phases out accordingly. When V_C reaches
the edge of the buck/boost range, at voltage V3, the AC
switch pair completely phase out the BD pair, and the boost
phase begins at duty cycle D4_SW. The input voltage, V_IN,
where the four switch region begins is given by:
V_IN = V_OUT(1 – D) = V_OUT(1 – 150ns • f) V
The point at which the four switch region ends is given
by:
V_IN
=
1−
V_OUT
(150ns
•
f)
V
where f = operating frequency, Hz.
Boost Region (V_IN < V_OUT)
Switch A is always on and switch B is always off during
this mode. When the control voltage, V_C, is above volt-
age 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 90% typical and is reached
when V_C is above V4.
BURST MODE OPERATION
Burst Mode operation reduces the LTC3533’s quiescent
current consumption at light loads and improves overall
conversion efficiency, increasing battery life. During Burst
Mode operation the LTC3533 delivers energy to the out-
put until it is regulated and then goes into a sleep mode
where the outputs are off and the quiescent current drops
to 40µA. 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. 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 rise to 450mA
typical and the inductor current will terminate at zero cur-
rent for each cycle. In this mode, the typical maximum
average output currents are given by:
I_{MAX(BURST)BUCK} ≈ 225mA; V_OUT < V_IN
I_{MAX(BURST)BOOST} ≈ 225mA • (V_IN/V_OUT); V_OUT > V_IN
I_{MAX(BURST)BUCK-BOOST} ≈ 350mA; V_OUT ≈ V_IN, since the
input and output are connected together for most of the
cycle.
The efficiency below 1mA becomes dominated primarily
by the quiescent current. The Burst Mode operation ef-
ficiency is given by:
Efficiency ≅ ^{η •ILOAD}
40µA + I_LOAD
where η is typically 90% during Burst Mode operation
Programmable Automatic Burst Mode Operation
Burst Mode operation can be automatic or digitally con-
trolled with a single pin. In automatic mode, the LTC3533
enters Burst Mode operation at the programmed threshold
and returns to fixed frequency operation when the load
demand increases. The load current at which the mode
transition occurs is programmed using a single external
resistor from BURST to ground, according to the follow-
ing equations:
Enter
Burst
Mode
Operation:
I_BURST
=
17
R_BURST
Exit
Burst
Mode
Operation:
I_BURST
=
19
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 1MΩ.
3533f
9