Datasheet
LTC3569
18
3569fe
For more information www.linear.com/LTC3569
Selecting The Inductors
Calculating the inductor values for 30% ripple current at
maximum SV
IN
:
L1 = 2.5V/(2.25MHz•240mA)•(1–2.5V/4.2V)= 1.9µH
L2 = 1.8V/(2.25MHz•120mA)•(1–1.8V/4.2V)= 3.8µH
L3 = 1.2V/(2.25MHz•120mA)•(1–1.2V/4.2V)= 3.1µH
Choosing a vendor’s closest values gives L1 = 2.2µH, L2
= L3 = 3.3µH. These values result in the maximum ripple
currents of:
∆I
L1
= 2.5V/(2.25MHz•2.2µH)•(1–2.5V/4.2V) = 204mA
∆I
L2
= 1.8V/(2.25MHz•3.3µH)•(1–1.8V/4.2V) = 139mA
∆I
L3
= 1.2V/(2.25MHz•3.3µH)•(1–1.2V/4.2V) = 115mA
Selecting The Output Capacitors
The value of the output capacitors are calculated based
on a 5% load droop for maximum load current step. The
output droop is usually about 2.5 times the linear drop
of the first cycle and is estimated based on the following
formula:
C
OUT
= 2.5•I
OUT(MAX)
/(f
CLK
•V
DROOP
)
The output capacitor values are calculated as:
C
OUT1
= 2.5•800mA/(2.25MHz•125mV) = 7.1µF
C
OUT2
= 2.5•400mA/(2.25MHz•90mV) = 4.9µF
C
OUT3
= 2.5•400mA/(2.25MHz•60mV) = 7.4µF
Choosing the closest standard values gives, C
OUT1
= 10µF,
C
OUT2
= 4.7µF and C
OUT3
= 10µF.
A 22µF input capacitor is selected since the Li-Ion battery
has sufficiently low output impedance.
Setting The Output Voltages
Without toggling the EN pins the LTC3569 develops a 0.8V
reference voltage for each of the feedback pins. The output
voltages are set by a resistive divider as follows:
V
OUT
= 0.8•(1 + R1/R2)
The resistors in Figure 5 are selected as the nearest 1%
standard resistor values. To improve frequency response
feedforward capacitors of 10pF and 20pF are used.
applicaTions inForMaTion
Figure 5. Triple Buck DC/DC Regulators: 800mA, 400mA, 400mA
Design Example 1: Burst Mode Operation
3569 F05a
SV
IN
PV
IN
SW1
PGNDSGND
SW2
SW3
PGOOD
FB1
EN1
EN2
EN3
R
T
MODE
FB2
FB3
22µF
V
IN
2.9V TO 4.2V
L1
2.2µH*
C
OUT1
10µF
10pF
20pF
C
OUT2
4.7µF
L2
3.3µH**
243k
115k
187k
150k
OUT1
2.5V AT 800mA
OUT2
1.8V AT 400mA
20pF
C
OUT3
10µF
L3
3.3µH**
187k
374k
OUT3
1.2V AT 400mA
LTC3569
470k
* WURTH 7447745022
** WURTH 7447745033
I
LOAD
(mA)
EFFICIENCY (%)
3569 F05b
100
90
80
70
60
50
0.1 1 1000 1000010010
BUCK1 = 2.5V
BUCK2 = 1.8V
BUCK3 = 1.2V
V
IN
= 2.9V TO 4.2V