Datasheet

LTC3828
15
3828fc
APPLICATIONS INFORMATION
to 1.1(INTV
CC
). The current comparator threshold sets the
peak of the inductor current, yielding a maximum average
output current I
MAX
equal to the peak value less half the
peak-to-peak ripple current, ΔI
L
.
Allowing a margin for variations in the IC and external
component values yields:
R
mV
I
SENSE
MAX
=
50
When using the controller in very low dropout conditions,
the maximum output current level will be reduced due to the
internal compensation required to meet stability criterion
for buck regulators operating at greater than 50% duty
factor. A curve is provided to estimate this reduction in
peak output current level depending upon the operating
duty factor.
Operating Frequency
The IC uses a constant frequency phase-lockable ar-
chitecture with the frequency determined by an internal
capacitor. This capacitor is charged by a fi xed current plus
an additional current which is proportional to the voltage
applied to the PLLFLTR pin. Refer to Phase-Locked Loop
and Frequency Synchronization in the Applications Infor-
mation section for additional information.
A graph for the voltage applied to the PLLFLTR pin vs
frequency is given in Figure 9. As the operating frequency
is increased the gate charge losses will be higher, reducing
effi ciency (see Effi ciency Considerations). The maximum
switching frequency is approximately 550kHz.
V
OSENSE1
C
SS
R3R1
R2
R5
V
OUT1
R2
R4
R2
R2
V
OSENSE2
V
OUT2
R5
R2
V
OUT4
R4
R2
V
OUT3
TRCKSS1
TRCKSS2
LTC3828
“MASTER”
TRCKSS1
V
OSENSE1
V
OSENSE2
TRCKSS2
LTC3828
“SLAVE”
(8a) Circuit Setup
TIME
3828 F08
(8b) Output Voltage
V
OUT1
V
OUT3
V
OUT4
V
OUT2
OUTPUT VOLTAGE
Figure 8. Four Outputs with Tracking and Ratiometric Sequencing
R
R
V
R
R
V
R
R
V
OUT OUT OUT
1
208
1
3
208
1
4
20
123
===
.
–,
.
–
.88
1
5
208
1
4
–,
.
–
R
R
V
OUT
=
⎛
⎝
⎜
⎞
⎠
⎟
OPERATING FREQUENCY (kHz)
200 300 400 500 600
PLLFLTR PIN VOLTAGE (V)
3828 F09
2.5
2.0
1.5
1.0
0.5
0
Figure 9. PLLFLTR Pin Voltage vs Frequency
Inductor Value Calculation
The operating frequency and inductor selection are inter-
related in that higher operating frequencies allow the use
of smaller inductor and capacitor values. So why would
anyone ever choose to operate at lower frequencies with
larger components? The answer is effi ciency. A higher
frequency generally results in lower effi ciency because