LTC3828 Datasheet, PDF (15/32 Pages) Linear Technology – Dual 2-Phase Step-Down Controller with Tracking

LTC3828
APPLICATIO S I FOR ATIO
V_OUT1
R4 R5 R1
TRCKSS2
LTC3828
“MASTER”
V_OUT2
R3
V_OSENSE1 V_OSENSE2
R2 R2 R2
TRCKSS1
R2
C_SS
V_OUT3
R4
R2
TRCKSS1 TRCKSS2
LTC3828
“SLAVE”
V_OSENSE1 V_OSENSE2
V_OUT4
R5
R2
(8a) Circuit Setup
V_OUT1
V_OUT3
V_OUT4
V_OUT2
TIME
3828 F08
(8b) Output Voltage
Figure 8. Four Outputs with Tracking and Ratiometric Sequencing
⎛ _R1
⎝⎜ R2
=
V_OUT1
0.8
– 1, ^R3
R2
=
V_OUT2
0.8
– 1R4
R2
=
V_OUT3
0.8
– 1, ^R5
R2
=
V_OUT4
0.8
⎞
– 1_⎠⎟
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, ∆I_L.
Allowing a margin for variations in the IC and external
component values yields:
R_SENSE
=
50mV
I_MAX
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 crite-
rion 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
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
efficiency (see Efficiency Considerations). The maximum
switching frequency is approximately 550kHz.
2.5
2.0
1.5
1.0
0.5
0
200
300
400
500
600
OPERATING FREQUENCY (kHz)
3828 F09
Figure 9. PLLFLTR Pin Voltage vs Frequency
The IC uses a constant frequency phase-lockable architec-
ture with the frequency determined by an internal capaci-
tor. This capacitor is charged by a fixed 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.
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 efficiency. A higher
frequency generally results in lower efficiency because of
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