LTC3856_15 Datasheet, PDF (29/40 Pages) Linear Technology – 2-Phase Synchronous Step-Down DC/DC Controller with Diffamp

LTC3856
Applications Information
Setting Output Voltage
If the DIFFAMP is not used, the LTC3856 output voltage
is set by an external feedback resistive divider carefully
placed across the output, as shown in Figure 14. The
regulated output voltage is determined by:
V_OUT
=
0.6V
•


1+
R_B
R_A


To improve the frequency response, a feedforward ca-
pacitor, C_FF , may be used. Great care should be taken to
route the V_FB line away from noise sources, such as the
inductor or the SW line.
If the diffamp is used, then V_FB should be connected to the
output of the diffamp, DIFFOUT, as shown in the Typical
Application on the first page.
V_OUT/ DIFFOUT
LTC3856
V_FB
R_B
C_FF
R_A
3856 F14
Figure 14. Setting Output Voltage without the DIFFAMP
Fault Conditions: Current Limit and Current Foldback
The LTC3856 includes current foldback to help limit load
current when the output is shorted to ground. If the out-
put falls below 50% of its nominal output level, then the
maximum sense voltage is progressively lowered from its
maximum programmed value to one-third of the maximum
value. Foldback current limiting is disabled during the
soft-start or tracking up. Under short-circuit conditions
with very low duty cycles, the LTC3856 will begin cycle
skipping in order to limit the short-circuit current. In this
situation the bottom MOSFET will be dissipating most of
the power but less than in normal operation. The short
circuit ripple current is determined by the minimum on-
time t_ON(MIN) of the LTC3856 (≈ 90ns), the input voltage
and inductor value:
∆I_L(SC)
=
t_ON(MIN)
•
V_IN
L
The resulting short-circuit current is:
I_SC
=



1/ 3
V_SENSE(MAX)
R_SENSE
–
1
2

∆I_L(SC)


•2
3856f
29