Datasheet

LTC3569
19
3569fe
For more information www.linear.com/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.
Figure 6. Dual Buck DC/DC Regulators: 1.8V at 1800mA, and 1.5V at 600mA
3569 F06a
PV
IN
SW1
PGNDSGND
SW2
SW3
PGOOD
MODE
SV
IN
FB1
EN1
EN2
EN3
R
T
FB2
FB3
22µF
0.1µF
V
IN
2.5V TO 5.5V
1µH
22µF20pF
20pF
2.2µH
187k
150k
174k
200k
10µF
OUT1
1.8V AT 1.8A
OUT3
1.5V AT 600mA
LTC3569
511k
V
IN
V
IN
Design Example 2: 1.8A Load Step on Buck1
with Buck2 Slave, Burst Mode Operation
CH1
SW1/2
2V/DIV
200mV/DIV
1A/DIV
1A/DIV
CH2
I
LOAD
1.8A
CH3
V
OUT1
336mV
P-P
CH4
I
L1
1.94A
10µs/DIV
3569 F06b