LTC3569_15 Datasheet, PDF (15/26 Pages) Linear Technology – Triple Buck Regulator with 1.2A and Two 600mA Outputs and Individual Programmable References

LTC3569
applications information
Setting the Output Voltages
The LTC3569 develops independent internal reference
voltages for each of the feedback pins. These reference
voltages are programmed from 0.8V down to 0.425V in
–25mV increments by toggling the appropriate EN pin.
The output voltage is set by a resistive divider according
to the following formula (refer to Figure 9 for resistor
designations):
V_OUT1 = V_REF1(1 + R1/R2),
where V_REF1 is programmed by toggling the EN1 pin.
V_OUT2 = V_REF2(1 + R3/R4),
where V_REF2 is programmed by toggling the EN2 pin.
V_OUT3 = V_REF3(1 + R5/R6),
where V_REF3 is programmed by toggling the EN3 pin.
Keeping the current small (<5µA) in these resistors
maximizes efficiency, but making the current too small
may allow stray capacitance to cause noise problems and
reduce the phase margin of the error amp loop.
To improve the frequency response, use a feedforward
capacitor, C_F, on the order of 20pF across the leading
feedback resistor (R1, R3, and R5). Take care to route
each FB line away from noise sources, such as the induc-
tor or the SW line. Remove the ground plane from below
the FB PCB routes to limit stray capacitance to GND on
these pins.
Inductor Selection
Although the inductor does not influence the operating
frequency, the inductor value has a direct effect on ripple
current. The inductor ripple current ∆I_L decreases with
higher inductance and increases with higher V_IN or V_OUT:
∆I_L = V_OUT/(f_CLK•L )•(1–V_OUT/V_IN)
Accepting larger values of ∆I_L allows the use of low
inductances, but results in higher output voltage ripple,
greater core losses, and lower output current capability.
A reasonable starting point for setting ripple current is
∆I_L = 0.3•I_OUT(MAX), where I_OUT(MAX) is the maximum
load current. The largest ripple current ∆I_L occurs at the
maximum input voltage. To guarantee that the ripple current
stays below a specified maximum, choose the inductor
value according to the following equation:
L = V_OUT/(f_CLK•∆I_L)•(1 – V_OUT/V_IN(MAX))
The inductor value also has an effect on Burst Mode
operation. The transition to low current operation begins
when the peak inductor current falls below a level set by
the burst clamp. Lower inductor values result in higher
ripple current which causes this to occur at lower load
currents. This causes a dip in efficiency in the upper
range of low current operation. In Burst Mode operation,
lower inductance values increase the burst frequency and
reduces efficiency.
Choose an inductor with a DC current rating at least 1.5
times larger than the maximum load current to ensure
that the inductor core does not saturate during normal
operation. If an output short-circuit is a possible condition,
select an inductor that is rated to handle the maximum
peak current specified for the regulators. To maximize
efficiency, choose an inductor with a low DC resistance;
as power loss in the inductor is due to I²R losses. Where
I² is the square of the average output current and R is the
ESR of the inductor.
Table 1. Low Profile Inductors
VENDOR/
PART NUMBER
VALUE
IDC
(µH) (APPROX.)
Wurth
7440430022
744031002
2.2
2.50
2.5
1.45
MuRata
LQH55PN1R2
LQH55PN2R2
1.2
2.60
2.2
2.10
Toko, DEV518C
1124BS-1R8N
1124BS-2R4M
1.8
2.70
2.4
2.30
EPCOS
B824691152M000 1.5
1.70
B824691221M000 2.2
1.55
RDC
(Ω)
0.023
0.050
0.021
0.031
0.047
0.054
0.046
0.065
HEIGHT
(mm)
2.80
1.65
1.85
1.85
1.80
1.80
1.20
1.20
For more information www.linear.com/LTC3569
3569fe
15