LT8708
43
Rev 0
Transconductance amplifier A3 performs this monitoring
function. A3 converts the current sense voltage, V
CSPIN-
CSNIN
, into two currents:
+
V
CSPIN-CSNIN
•1m
A
V
and
– V
CSPIN-CSNIN
•1m
A
V
These currents are added to 20μA offsets and then forced
into R
IMON_INP
and R
IMON_INN
, respectively.
Due to the 20μA offset currents, V
IMON_INP
and V
IMON_
INN
are not 0V when I
IN
is 0A. Instead, V
IMON_INP(0)
=
20μA•R
IMON_INP
Volts and V
IMON_INN(0)
= 20μA•R
IMON_INN
Volts (typical) when I
IN
= 0 Amps. As I
IN
becomes increas-
ingly negative, V
IMON_INP
reduces below V
IMON_INP(0)
until V
IMON_INP
= 0V. Similarly, as I
IN
becomes increas-
ingly positive, V
IMON_INN
reduces below V
IMON_INN(0)
until V
IMON_INN
= 0V. I
MON_INP
and I
MON_INN
will not be
driven below ground as their output currents can only be
positive or zero.
The complete transfer functions for IMON_INP and
IMON_INN are given in the equations below:
V
IMON _INP
=
1m
A
V
•R
SENSE1
•I
IN
+ 20µA
⎛
⎝
⎜
⎜
⎞
⎠
⎟
⎟
•R
IMON _INP
V
IMON _INN
=
–1m
A
V
•R
SENSE1
•I
IN
+ 20µA
⎛
⎝
⎜
⎜
⎞
⎠
⎟
⎟
•R
IMON _INN
The differential voltage V
CSPIN-CSNIN
should remain be-
tween –100mV and 100mV due to the limited current that
can be driven out of IMON_INP and IMON_INN. If the
instantaneous V
CSPIN-CSNIN
exceeds these limits but the
average V
CSPIN-CSNIN
is within the limits, consider includ-
ing the current sense filter described in the next section.
In addition, IMON_INP and IMON_INN should be filtered
with capacitors C
IMON_INP
and C
IMON_INN
due to I
IN
ripple
and discontinuities that can occur in various regions of
operation. A few nF of capacitance is usually sufficient.
APPLICATIONS INFORMATION
Current Limiting: As shown in Figure 15, IMON_INP voltage
that exceeds 1.209V (typical) causes V
C
to reduce, thus
limiting the forward I
IN
and inductor currents. IMON_INN
voltage that exceeds 1.21V (typical) causes V
C
to increase,
thus limiting the reverse I
IN
and inductor currents (see the
Error Amplifiers section).
The forward I
IN
limit, I
(IN,FWD,LIMIT)
, can be set as needed
by choosing the appropriate R
SENSE1
and R
IMON_INP
resis-
tors using the following equation:
R
IMON _INP
=
1.209
I
(IN,FWD,LIMIT)
• 1m
A
V
•R
SENSE1
+ 20µA
Ω
For example, if R
SENSE1
is chosen to be 12.5mΩ and the
desired forward I
IN
current limit is 4A then:
R
IMON _INP
=
1.209
4A • 1m
A
V
• 12.5mΩ + 20µA
=
17.3kΩ
Similarly, the reverse I
IN
limit, I
(IN,RVS,LIMIT)
, can be set as
needed by choosing the appropriate R
SENSE1
and R
IMON_INN
resistors using the following equation:
R
IMON _INN
=
1.21
I
(IN,RVS,LIMIT)
•1m
A
V
•R
ESENSE1
+ 20µA
Ω
C
IMON_INP
and C
IMON_INN
capacitors of at least a few nF are
necessary to maintain loop stability when IMON_INP and
IMON_INN, respectively, are used to operate the LT8708
at constant current limit.
Review the Electrical Characteristics and the IMON Output
Currents graph in the Typical Performance Characteris-
tics section to understand the operational limits of the
IMON_OP, IMON_ON, IMON_INP and IMON_INN currents.
External currents can be summed to the IMON pins to
adjust I
IN
and/or I
OUT
limit in both directions while switch-
ing. When the IMON_OP and IMON_ON pins are used in
