LT4180_15 Datasheet, PDF (8/18 Pages) Linear Technology – Virtual Remote Sense Controller

LT4180
Operation
from degrading contacts. Making the capacitor larger can
minimize the voltage ripple at the load due to a combination
of load regulation and the dither frequency of the LT4180.
Figure 3 shows the timing diagram for Virtual Remote
Sense. A new cycle begins when the power supply and
Virtual Remote Sense close the loop around V_OUT (regulate
V_OUT = H). Both V_OUT and I_OUT slew and settle to a new
value, and these values are stored in the Virtual Remote
Sense (track V_OUT high = L and track I_OUT = L). The V_OUT
feedback loop is opened and a new feedback loop is set
up commanding the power supply to deliver 90% of the
previously measured current (0.9I_OUT). V_OUT drops to a new
value as the power supply reaches a new steady state, and
this information is also stored in the Virtual Remote Sense.
At this point, the change in output voltage (∆V_OUT) for a
–10% change in output current has been measured and
is stored in the Virtual Remote Sense. This voltage is used
during the next Virtual Remote Sense cycle to compensate
for voltage drops due to wiring resistance.
V_OUT
REGULATE V_OUT
TRACK V_OUT HIGH
TRACK I_OUT
REGULATE I_OUT LOW
TRACK V_OUT LOW
TRACK ∆V_OUT
4180 F03
Figure 3. Simplified Timing Diagram, Virtual Remote Sense
Applications Information
INTRODUCTION
The LT4180 is designed to interface with a variety of power
supplies and regulators having either an external feedback
or control pin. In Figure 4, the regulator error amplifier
(which is a g_m amplifier) is disabled by tying its inverting
input to ground. This converts the error amplifier into a
constant-current source which is then controlled by the
drain pin of the LT4180. This is the preferred method of
interfacing because it eliminates the regulator error ampli-
fier from the control loop which simplifies compensation
and provides best control loop response.
REGULATOR
+
I_TH OR
V_C
–
LT4180
DRAIN
Isolated power supplies and regulators may also be used
by adding an opto-coupler (Figure 5). LT4180 output volt-
age INTV_CC supplies power to the opto-coupler LED. In
situations where the control pin V_C of the regulator may
exceed 5V, a cascode may be added to keep the DRAIN
pin of the LT4180 below 5V (Figure 6). Use a low VT
MOSFET for the cascode transistor.
REGULATOR
+ V_C
–
OPTO-COUPLER
INTV_CC
LT4180
DRAIN
4180 F05
Figure 5. Isolated Power Supply Interface
4180 F04
Figure 4. Nonisolated Regulator Interface
For proper operation, increasing control voltage should
correspond to increasing regulator output. For example,
in the case of a current mode switching power supply,
the control pin ITH should produce higher peak currents
as the ITH pin voltage is made more positive.
TO V_C > 5V
COMP
LT4180
INTV_CC
DRAIN
4180 F06
Figure 6. Cascoded DRAIN Pin for Isolated Supplies
4180fb
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