LTC3532_15 Datasheet, PDF (9/16 Pages) Linear Technology – Micropower Synchronous Buck-Boost DC/DC Converter

U
OPERATIO
V_IN
V_OUT
7
6
A
dI
dt
≈
V_IN
L
D
+
–
3
4
L
SW1
SW2
B
C
250mA
0mA
T1
5
GND
LTC3532
3532 F03
V_IN
V_OUT
7
6
A
dI
dt
≈
–
V_OUT
L
D
–
+
3
4
SW1
L
SW2
B
C
250mA
0mA
5
GND
T2
3532 F04
Figure 3. Inductor Charge Cycle During Burst Mode Operation Figure 4. Inductor Disharge Cycle During Burst Mode Operation
Note that the peak efficiency during Burst Mode operation
is less than the peak efficiency during fixed frequency
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 • ILOAD}
35μA + I_LOAD
where n is typically 88% 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 Operation: I = ^10.5V
R_BURST
Leave Burst Mode Operation: I = ^7V
R_BURST
where R_BURST is in kΩ and I_BURST is the load transition
current in Amps. For automatic operation, a filter capaci-
tor should also be connected from BURST to ground to
prevent ripple on BURST from causing the IC to oscillate
in and out of Burst Mode operation. The equation for the
minimum capacitor value is:
C_{B U R S T(M I N)}
≥
C_OUT • V_OUT
60,000V
where C_BURST(MIN) and C_OUT are in μF. In the event that
a load transient causes the feedback pin to drop by more
than 4% from the regulation value while in Burst Mode
operation, the IC will immediately switch to fixed frequency
mode and an internal pull-up will be momentarily applied
to BURST, rapidly charging the BURST capacitor. This
prevents the IC from immediately reentering Burst Mode
operation once the output achieves regulation.
Manual Burst Mode Operation
For manual control of Burst Mode operation, the RC net-
work connected to BURST can be eliminated. To force fixed
frequency mode, BURST should be connected to V_OUT. To
force Burst Mode operation, BURST should be grounded.
When commanding Burst Mode operation manually, the
circuit connected to BURST should be able to sink up to
2mA. For optimum transient response with large dynamic
loads, the operating mode should be controlled manually
by the host. By commanding fixed frequency operation
prior to a sudden increase in load, output voltage droop can
3532fc
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