Linear Technology Analog Devices LT8708, Техническое описание

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LT8708
44
Rev 0
For more information www.analog.com
Figure 17. CSPIN/CSNIN and CSPOUT/CSNOUT
Current Sense Filter
CSPIN CSNIN
LT8708
C
FILTER1
R
FILTER1
R
SENSE1
CSPOUT CSNOUT
LT8708
8708 F17
C
FILTER2
R
FILTER2
R
SENSE2
this way, ICP and ICN can be used to monitor the I
OUT
current in the forward and reverse directions respectively
(see the Current Monitoring, Regulation and Limiting: ICP
and ICN Pins section).
Current Sense Filter: The + and – outputs of current sense
amplifiers A1 and A3 are rated to provide a range of –20μA to
+100μA. For example, IMON_INP, which primarily reports
forward I
IN
current, may not provide the expected output
current when V
CSPIN-CSNIN
exceeds 100mV. In addition,
the IMON_INP pin will not provide the expected output
current when V
CSPIN-CSNIN
is below –20mV.
Currents that flow through the current sense resistors
(R
SENSE1
, R
SENSE2
in Figure 17) are often discontinu-
ous and can contain significant AC content during each
switching cycle. One example is the forward I
IN
in the buck
region. If the I
IN
current presents an average differential
(V
CSPIN-CSNIN
) less than 100mV, but contains AC peaks
exceeding 100mV, the IMON_INP current may clip. To
prevent clipping, the current sense filter shown in Figure
17, can be added. The filter will reduce the peak differ-
ential (V
CSPIN-CSNIN
) to <100mV while keeping the same
average, thus allowing the correct result to be presented
on IMON_INP. As another example, consider the reverse
I
OUT
measured by IMON_ON. If the current presents an
average differential (V
CSNOUT-CSPOUT
) less than 100mV, but
contains AC peaks exceeding 100mV, the current sense
filter can be used to reduce the peaks below 100mV while
keeping the same average.
The –20μA output current limits for amplifiers A1 and A3
are often most important when using the HCM mode (see
the Unidirectional Conduction: HCM section). The current
sense amplifier outputs may clip at the –20μA limits when
the average sensed current is low but contains high AC
content. Clipping may distort the ICN or IMON_INP voltages
that are used to select between heavy and light load HCM
operation. Once again, the current sense filter can be used
to reduce the AC content appearing at the amplifier inputs.
Current sense filter(s) should be connected as shown in
Figure 16. Note that resistance in series with CSNIN and
CSNOUT is not recommended. As described in the Topside
MOSFET Driver Supply (CB1, DB1, CB2, DB2) section, the
CSNIN and CSNOUT pins are also connected to the Boost
Cap Charge Control block (also see Figure 1) and can draw
current under certain conditions. In addition, the same
CSNIN and CSNOUT current sense pins can draw bias cur-
rent under normal operating conditions, while CSPIN and
CSPOUT draw zero (typical) bias current. A time constant
lower than 10μs is recommended for the filter(s).
Also, because of their use with the Boost Cap Charge
Control block, tie the CSPIN and CSNIN pins to V
IN
and
tie the IMON_INP and IMON_INN pins to ground when the
input current sensing is not in use. Similarly, the CSPOUT
and CSNOUT pins should be tied to V
OUT
, the IMON_OP,
IMON_ON pins should be grounded when not in use.
LOOP COMPENSATION
The loop stability is affected by a number of factors includ-
ing the inductor value, output capacitance, load current,
V
IN
, V
OUT
and the V
C
resistor and capacitors. The LT8708
uses internal transconductance error amplifiers driving V
C
to help compensate the control loop. For most applications
a 3.3nF series capacitor at V
C
is a good value. The parallel
capacitor (from V
C
to GND) is typically 1/10th the value
of the series capacitor to filter high frequency noise. A
larger V
C
series capacitor value may be necessary if the
output capacitance is reduced. A good starting value for
the V
C
series resistor is 20k. Lower resistance will improve
stability but will slow the loop response. Use a trim pot
instead of a fixed resistor for initial bench evaluation to
determine the optimum value.
Also note that C
IMON_INP
and C
IMON_INN
capacitors of at
least a few nF are necessary to maintain loop stability
APPLICATIONS INFORMATION