LTC3569_15 Datasheet, PDF (19/26 Pages) Linear Technology – Triple Buck Regulator with 1.2A and Two 600mA Outputs and Individual Programmable References

LTC3569
Applications Information
Design Example 2: Dual Bucks, 1.8V at 1.8A and 1.5V
at 600mA
For this example, the LTC3569 is configured to deliver two
fixed voltages of 1.8V and 1.5V from a generic supply over
the full operating range, 2.5V to 5.5V. The load require-
ments range from <1mA in standby mode up to 1.8A for
the 1.8V supply and 600mA for the 1.5V supply.
The fixed internal clock frequency of 2.25MHz meets the
minimum on-time requirements. Burst Mode operation
is selected for high efficiency at the low standby current
level. Calculating the inductor values for 30% ripple cur-
rent at max SV_IN:
L1 = 1.8V/(2.25MHz•540mA)•(1–1.8V/5.5V) = 1.0µH
L2 = 1.5V/(2.25MHz•180mA)•(1–1.5V/5.5V) = 2.2µH
Calculating the value of the output capacitors:
C_OUT1 = 2.5•1800mA/(2.25MHz•90mV) = 22µF
C_OUT2 = 2.5•600mA/(2.25MHz•75mV) = 8.9µF
An input capacitor of 22µF is selected to support the
maximum ripple current of 1.2A. An additional 0.1µF low
ESR capacitor is placed between SV_IN and SGND.
The resistor values shown in Figure 6 are selected as the
closest standard 1% resistors to obtain the correct output
voltages with the full-scale reference voltages of 0.8V. And
20pF feedforward capacitors are placed across the leading
feedback resistors.
V_IN 2.5V TO 5.5V
22µF
PV_IN SW1
SW2
EN1
FB1
EN2
LTC3569
EN3
R_T
FB2
MODE
511k
SW3
PGOOD
V_IN
SV_IN
FB3
0.1µF _{SGND PGND}
1µH
187k
150k
V_IN
2.2µH
174k
200k
OUT1
1.8V AT 1.8A
20pF
22µF
OUT3
1.5V AT 600mA
20pF
10µF
3569 F06a
Design Example 2: 1.8A Load Step on Buck1
with Buck2 Slave, Burst Mode Operation
CH1
SW1/2
CH3
V_OUT1
336mV_P-P
CH2
I_LOAD
1.8A
CH4
I_L1
1.94A
2V/DIV
200mV/DIV
10µs/DIV
1A/DIV
1A/DIV
3569 F06b
Figure 6. Dual Buck DC/DC Regulators: 1.8V at 1800mA, and 1.5V at 600mA
For more information www.linear.com/LTC3569
3569fe
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