Datasheet
LTC3569
20
3569fe
For more information www.linear.com/LTC3569
applicaTions inForMaTion
Figure 7. Dual Programmable Buck DC/DC Regulators: 1200mA, 1200mA
3569 F07a
SV
IN
PV
IN
SW1
PGNDSGND
SW2
SW3
PGOOD
FB1
EN1
EN2
EN3
MODE
R
T
FB2
FB3
22µF
V
IN
2.5V TO 5.5V
1.5µH
15µF20pF
20pF
15µF
1.5µH
294k
210k
294k
210k
OUT1 1200mA
1.2V STANDBY
1.8V ACTIVE
OUT2 1200mA
1.2V STANDBY
1.8V ACTIVE
LTC3569
511k
V
IN
V
IN
DIGITAL
CONTROL
Design Example 3: Soft-Start to Standby (1.2V)
Design Example 3: Dual Programmable Bucks
In this example consider two buck regulators operating
from a 2.5V to 5.5V unregulated supply that are required
to generate two independently programmable supplies that
must step from 1.2V in standby up to 1.8V when active,
with a maximum load current of 1.2A when active and
1mA in standby. Additionally, this application anticipates
possible output short circuits, and is required to operate
without damage in such a situation.
Buck 1 is selected for the first regulator, and buck 3 is
configured as a slave power stage in parallel with buck 2
by pulling FB3 up to V
IN
to obtain the required current level
for the second regulator. Burst Mode operation is selected
to achieve high efficiency during standby operation. The
internal 2.25MHz clock frequency is selected, as it satisfies
the minimum on-time requirement. Next, two reference
voltages are selected to match the ratio of the active to
standby voltages: 1.8V/1.2V = 1.5. The 0.75V and 0.5V
reference levels mach this ratio. The resistors shown in
Figure 7 are selected to obtain the correct feedback ratio
from standard 1% resistors. Calculating the inductor values
for 30% ripple current at maximum SV
IN
:
L = 1.8V/(2.25MHz•360mA)•(1–1.8V/5.5V)= 1.5µH.
The output capacitor values are selected as the nearest
standard value to obtain 5% voltage droop at maximum
load current step.
C
OUT
= 2.5•1200mA/(2.25MHz•90mV) ≈ 15µF.
Select an output capacitor with an ESR of less than 50mΩ
to obtain an output voltage ripple of less than 30mV.
Finally select an input capacitor rated for the worst-case
short-circuit ripple current of 2 I
PK
/√3 ≈ 2.5A, when both
outputs are shorted to GND.
CH1
OUT1
CH3
PGOOD
CH2
OUT2
CH4
EN1 = EN2
200µs/DIV
3569 F07b
500mV/DIV
500mV/DIV
2V/DIV
2V/DIV