LTC3736-1 Datasheet, PDF (19/28 Pages) Linear Technology – Dual 2-Phase, No RSENSE Synchronous Controller with Spread Spectrum

LTC3736-1
APPLICATIO S I FOR ATIO
The ramp time for V_OUT2 to rise from 0V to its final value
is:
t_SS2
=
t_SS1
•
R_TRACKA
R1A
•
R1A + R1B
R_TRACKA + R_TRACKB
For coincident tracking,
t_SS2
=
t_SS1
•
V_OUT2F
V_OUT1F
where V_OUT1F and V_OUT2F are the final, regulated values of
V_OUT1 and V_OUT2. V_OUT1 should always be greater than
V_OUT1
V_OUT2
R_TRACKB
R_TRACKA
R1B LTC3736-1
R2B
V_FB1 V_FB2
R1A
R2A
TRACK
37361 F08a
Figure 8a. Using the TRACK Pin
V_OUT1
V_OUT2
V_OUT2 when using the TRACK pin. If no tracking function
is desired, then the TRACK pin may be tied to V_IN. How-
ever, in this situation there would be no (internal nor
external) soft-start on V_OUT2.
When using tracking with spread spectrum operation
enabled, the tracking resistors R_TRACKA and R_TRACKB
should have value at least 10 times smaller than corre-
sponding feedback resistors R2A and R2B.
Fault Condition: Short Circuit and Current Limit
To prevent excessive heating of the bottom MOSFET,
foldback current limiting can be added to reduce the
current in proportion to the severity of the fault.
Foldback current limiting is implemented by adding di-
odes D_FB1 and D_FB2 between the output and the I_TH pin as
shown in Figure 9. In a hard short (V_OUT = 0V), the current
will be reduced to approximately 50% of the maximum
output current.
Low Supply Operation
Although the LTC3736-1 can function down to below
2.4V, the maximum allowable output current is reduced as
V_IN decreases below 3V. Figure 10 shows the amount of
1/2 LTC3736-1
I_TH
V_FB
R2 ₊
R1
V_OUT
D_FB1
D_FB2
TIME
(8b) Coincident Tracking
V_OUT1
V_OUT2
TIME
37361 F08b,c
(8c) Ratiometric Tracking
Figures 8b and 8c. Two Different Modes of Output
Voltage Tracking
37361 F09
Figure 9. Foldback Current Limiting
105
V_REF
100
95
MAXIMUM
SENSE VOLTAGE
90
85
80
75
2.0 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 3.0
INPUT VOLTAGE (V)
37361 F10
Figure 10. Line Regulation of V_REF and
Maximum Sense Voltage for Low Input Supply
37361f
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