CS5302
considered when setting the I
threshold. If a more
LIM
accurate current sense is required than inductive sensing can
provide, current can be sensed through a resistor as shown
in Figure 7.
SWNODE
Current Sharing Accuracy
PCB traces that carry inductor current can be used as part
of the current sense resistance depending on where the
current sense signal is picked off. For accurate current
sharing, the current sense inputs should sense the current at
the same point for each phase and the connection to the
V
FB
(V
)
OUT
CSA Out
CS
should be made so that no phase is favored. (In some
REF
COMP – Offset
CSA Out + V
cases, especially with inductive sensing, resistance of the
pcb can be useful for increasing the current sense
resistance.) The total current sense resistance used for
calculations must include any pcb trace between the CS
FB
T1
T2
Figure 8. Open Loop Operation
inputs and the CS
input that carries inductor current.
REF
Current Sense Amplifier Input Mismatch and the value of
the current sense element will determine the accuracy of
current sharing between phases. The worst case Current
Sense Amplifier Input Mismatch is 5.0 mV and will
typically be within 3.0 mV. The difference in peak currents
between phases will be the CSA Input Mismatch divided by
the current sense resistance. If all current sense elements are
of equal resistance a 3.0 mV mismatch with a 2.0 mΩ sense
resistance will produce a 1.5 A difference in current between
phases.
Inductive Current Sensing
For lossless sensing, current can be sensed across the
inductor as shown in Figure 9. In the diagram L is the output
inductance and R is the inherent inductor resistance. To
L
compensate the current sense signal the values of R1 and C1
are chosen so that L/R = R1 × C1. If this criteria is met the
L
current sense signal will be the same shape as the inductor
current, the voltage signal at Cx will represent the
instantaneous value of inductor current and the circuit can be
analyzed as if a sense resistor of value R was used as a sense
L
Operation at > 50% Duty Cycle
For operation at duty cycles above 50% Enhanced V will
resistor (R ).
S
2
R1
C1
exhibit subharmonic oscillation unless a compensation
ramp is added to each phase. A circuit like the one on the left
side of Figure 10 can be added to each current sense network
to implement slope compensation. The value of R1 can be
varied to adjust the ramp size.
SWNODE
CS
CS
+
L
+
+
+
CSA
OFFSET
R
L
REF
PWM
COMP
+
Switch Node
Gate(L)X
V
OUT
V
FB
E.A.
+
DAC
OUT
COMP
R1
25 k
3 k
CS
Figure 9. Lossless Inductive Current Sensing with
Enhanced V2
X
1.0 nF
0.1 µF
.01 µF
When choosing or designing inductors for use with
inductive sensing tolerances and temperature, effects should
be considered. Cores with a low permeability material or a
large gap will usually have minimal inductance change with
temperature and load. Copper magnet wire has a
temperature coefficient of 0.39% per °C. The increase in
winding resistance at higher temperatures should be
CS
REF
MMBT2222LT1
Existing Current
Sense Circuit
Slope Comp
Circuit
Figure 10. External Slope Compensation Circuit
http://onsemi.com
11
CHERRY [ CHERRY SEMICONDUCTOR CORPORATION ]
