LTC3569_15 Datasheet, PDF (18/26 Pages) Linear Technology – Triple Buck Regulator with 1.2A and Two 600mA Outputs and Individual Programmable References

LTC3569
applications information
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:
ΔI_L1 = 2.5V/(2.25MHz•2.2µH)•(1–2.5V/4.2V) = 204mA
ΔI_L2 = 1.8V/(2.25MHz•3.3µH)•(1–1.8V/4.2V) = 139mA
ΔI_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.
V_IN 2.9V TO 4.2V
22µF _{SVIN PVIN SW1}
EN1
FB1
EN2
LTC3569
EN3
R_T
SW2
MODE
470k
FB2
PGOOD
SW3
FB3
SGND PGND
L1
2.2µH*
243k
115k
L2
3.3µH**
187k
150k
L3
3.3µH**
187k
374k
* WURTH 7447745022
** WURTH 7447745033
10pF
20pF
20pF
OUT1
2.5V AT 800mA
C_OUT1
10µF
OUT2
1.8V AT 400mA
C_OUT2
4.7µF
OUT3
1.2V AT 400mA
C_OUT3
10µF
3569 F05a
Design Example 1: Burst Mode Operation
100
V_IN = 2.9V TO 4.2V
90
80
70
60
BUCK1 = 2.5V
BUCK2 = 1.8V
BUCK3 = 1.2V
50
0.1
1
10 100 1000 10000
I_LOAD (mA)
3569 F05b
Figure 5. Triple Buck DC/DC Regulators: 800mA, 400mA, 400mA
3569fe
18
For more information www.linear.com/LTC3569