NCP81119
http://onsemi.com
20
Total Current Sense Amplifier
The NCP81119 uses a patented approach to sum the phase currents into a single temperature compensated total current signal.
This signal is then used to generate the output voltage droop, total current limit, and the output current monitoring functions.
The total current signal is floating with respect to CSREF. The current signal is the difference between CSCOMP and CSREF.
The Ref(n) resistors sum the signals from the output side of the inductors to create a low impedance virtual ground, the capacitor
is used to ensure that the CSREF voltage signal integrity. The amplifier actively filters and gains up the voltage applied across
the inductors to recover the voltage drop across the inductor series resistance (DCR). Rth is placed near an inductor to sense
the temperature of the inductor. This allows the filter time constant and gain to be a function of the Rth NTC resistor and
compensate for the change in the DCR with temperature.
−
+
U1A
Rph4
Rph3
Rph2
Rph1
Rref4 10
Rfer3 10
Rref2 10
Rref1 10
R9
R
R10
R
Cref
1nF
Ccs1
Ccs2
t
RT1
100k
CSREF
CSN2
CSN1
CSN4
CSN3
SWN2
SWN1
SWN4
SWN3
0
CSSUM
CSCOMP
Figure 7.
The DC gain equation for the current sensing:
V
CSCOMP−CSREF
+
Rcs2 )
Rcs1*Rth
Rcs1)Rth
Rph
*
ǒ
Iout
Total
* DCR
Ǔ
Set the gain by adjusting the value of the Rph resistors. The DC gain should be set to the output voltage droop. If the voltage
from CSCOMP to CSREF is less than 100 mV at ICCMAX then it is recommend increasing the gain of the CSCOMP amp.
This is required to provide a good current signal to offset voltage ratio for the ILIMIT pin. When no droop is needed, the gain
of the amplifier should be set to provide ~100mV across the current limit programming resistor at full load. The values of Rcs1
and Rcs2 are set based on the 100k NTC and the temperature effect of the inductor and should not need to be changed. The
NTC should be placed near the closest inductor. The output voltage droop should be set with the droop filter divider.
The pole frequency in the CSCOMP filter should be set equal to the zero from the output inductor. This allows the circuit
to recover the inductor DCR voltage drop current signal. Ccs1 and Ccs2 are in parallel to allow for fine tuning of the time
constant using commonly available values. It is best to fine tune this filter during transient testing.
F
z
+
DCR@25° C
2*PI*L
Phase
Programming the Current Limit
The current limit thresholds are programmed with a resistor between the ILIMIT and CSCOMP pins. The ILIMIT pin mirrors
the voltage at the CSREF pin and mirrors the sink current internally to IOUT (reduced by the IOUT Current Gain) and the
current limit comparators. The 100% current limit trips if the ILIMIT sink current exceeds 10 mA for 50 ms. The 150% current
limit trips with minimal delay if the ILIMIT sink current exceeds 15 mA. Set the value of the current limit resistor based on
the CSCOMP−CSREF voltage as shown below.
R
LIMIT
+
Rcs2)
Rcs1*Rth
Rcs1)Rth
Rph
*
ǒ
Iout
LIMIT
* DCR
Ǔ
10m
or R
LIMIT
+
lV
CSCOMP−CSREF@ILIMIT
10m
