LTC3532 LINER [Linear Technology], LTC3532 Datasheet - Page 9

• Manufacturer Part Number: LTC3532
• Description: Micropower Synchronous Buck-Boost DC/DC Converter
• Manufacturer: LINER [Linear Technology]

OPERATIO
Note that the peak effi ciency during Burst Mode operation
is less than the peak effi ciency during fi xed 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 effi ciency below
1mA becomes dominated primarily by the quiescent cur-
rent. The Burst Mode operation effi ciency is given by:
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 fi xed
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:
A
B
V
7
Figure 3. Inductor Charge Cycle During Burst Mode Operation
IN
EFFICIENCY ≅
Enter Burst Mode Operation: I =
Leave Burst Mode Operation: I =
SW1
3
+
dI
dt
GND
≈
L
5
V
L
IN
–
SW2
U
4
35μA + I
D
V
C
n • I
OUT
6
LOAD
250mA
LOAD
0mA
T1
R
R
10.5V
BURST
BURST
7V
3532 F03
Figure 4. Inductor Disharge Cycle During Burst Mode Operation
A
B
where R
current in Amps. For automatic operation, a fi lter 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:
where C
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 fi xed 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 fi xed
frequency mode, BURST should be connected to V
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 fi xed frequency operation
prior to a sudden increase in load, output voltage droop can
V
7
IN
C
SW1
BURST(MIN)
3
–
BURST(MIN)
BURST
dI
dt
≈ –
GND
L
5
V
OUT
L
is in kΩ and I
+
≥
SW2
4
C
and C
OUT
60,000V
D
C
V
OUT
6
• V
OUT
OUT
250mA
BURST
0mA
are in μF . In the event that
is the load transition
LTC3532
T2
OUT
3532 F04
3532fc
9
. To