CS5323

http://onsemi.com

10

SWNODE

CSA Out

Figure 6. Open Loop Operation

COMP – Offset

T1

T2

Inductive Current Sensing

For lossless sensing current can be sensed across the

inductor as shown below in Figure 7. In the diagram, L is the

To compensate the current sense signal the values of R1 and

the 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

SWNODE

COMP

CSA

OFFSET

PWM

COMP

E.A.

R1

C1

L

+

+

+

+

+

+

Figure 7. Lossless Inductive Current Sensing with

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

accurate current sense is required than inductive sensing can

provide, current can be sensed through a resistor as shown

in Figure 5.

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

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

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 mV and will typically

be within 3 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 mV mismatch with a 2 m

Ω sense

resistance will produce a 1.5 A difference in current between

phases.

Operation at > 50% Duty Cycle

will exhibit subharmonic oscillation unless a compensation

ramp is added to each phase. A circuit like the one on the left

side of Figure 8 can be added to each current sense network

to implement slope compensation. The value of R1 can be

varied to adjust the ramp size.

Switch Node

25 k

R1

.01

1.0 nF

0.1

3 k

GATE(L)X

Slope Comp

Circuit

Existing Current

Sense Circuit

MMBT2222LT1

Figure 8. External Slope Compensation Circuit