LTC3569_15 Datasheet, PDF (20/26 Pages) Linear Technology – Triple Buck Regulator with 1.2A and Two 600mA Outputs and Individual Programmable References

LTC3569
applications information
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.
V_IN 2.5V TO 5.5V
22µF _{SVIN PVIN SW1}
DIGITAL
CONTROL
V_IN
EN1
FB1
EN2
LTC3569
EN3
MODE
SW2
R_T
SW3
511k
FB2
PGOOD
FB3
SGND PGND
1.5µH
294k
210k
1.5µH
294k
210k
V_IN
20pF
20pF
OUT1 1200mA
1.2V STANDBY
15µF ^{1.8V ACTIVE}
OUT2 1200mA
1.2V STANDBY
15µF ^{1.8V ACTIVE}
3569 F07a
Design Example 3: Soft-Start to Standby (1.2V)
CH1
OUT1
CH2
OUT2
CH3
PGOOD
CH4
EN1 = EN2
200µs/DIV
500mV/DIV
500mV/DIV
2V/DIV
2V/DIV
3569 F07b
Figure 7. Dual Programmable Buck DC/DC Regulators: 1200mA, 1200mA
3569fe
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