ZL2004 datasheet(22/42 Pages) RENESAS

22 / 42 page

ZL2004

FN6846 Rev. 3.00

Page 22 of 42

February 15, 2011

5.7.9 Current Sensing Components

Once the current sense method has been selected

(Refer to Section 5.8, “Current Limit Threshold

Selection,”), the components are selected as follows.

When using the inductor DCR sensing method, the

user must also select an R/C network comprised of R1

and CL (see Figure 14).

VIN

VOUT

PWMH

ISENA

ISENB

PWML

Figure 14. DCR Current Sensing

For the voltage across CL to reflect the voltage across

the DCR of the inductor, the time constant of the

inductor must match the time constant of the RC

network. That is:

DCR









For L, use the average of the nominal value and the

minimum value. Include the effects of tolerance, DC

Bias and switching frequency on the inductance when

determining the minimum value of L. Use the typical

value for DCR.

The value of R1 should be as small as feasible and no

greater than 5 kΩ for best signal-to-noise ratio. The

designer should make sure the resistor package size is

appropriate for the power dissipated and include this

loss in efficiency calculations. In calculating the

minimum value of R1, the average voltage across CL

(which is the average IOUT·DCR product) is small and

can be neglected. Therefore, the minimum value of R1

may be approximated by the following equation:



 



pkg

OUT















max

min

where PR1pkg-max is the maximum power dissipation

specification for the resistor package and δP is the

derating factor for the same parameter (eg.: PR1pkg-max =

0.0625W for 0603 package, δP = 50% @ 85°C). Once

R1-min has been calculated, solve for the maximum

value of CL from

DCR







min

max

and choose the next-lowest readily available value (eg.:

For CL-max = 1.86µF, CL = 1.5µF is a good choice).

Then substitute the chosen value into the same

equation and re-calculate the value of R1. Choose the

1% resistor standard value closest to this re-calculated

value of R1. The error due to the mismatch of the two

time constants is

100





















avg

DCR





The value of R2 should be simply five times that of R1:

5 R





5.8 Current Limit Threshold Selection

It is recommended that the user include a current

limiting mechanism in their design to protect the power

supply from damage and prevent excessive current

from being drawn from the input supply in the event

that the output is shorted to ground or an overload

condition is imposed on the output. Current limiting is

accomplished by sensing the current through the circuit

during a portion of the duty cycle.

Output current sensing can be accomplished by

measuring the voltage across a series resistive sensing

element according to the following equation:

SENSE

LIM





Where:

ILIM is the desired maximum current that should

flow in the circuit

RSENSE is the resistance of the sensing element

VLIM is the voltage across the sensing element at the

point the circuit should start limiting the output

current.

The ZL2004 supports “lossless” current sensing, by

measuring the voltage across a resistive element that is

already

present

the

circuit.

This

eliminates

additional efficiency losses incurred by devices that

must use an additional series resistance in the circuit.

To set the current limit threshold, the user must first

select

current

sensing

method.

The

ZL2004

incorporates one method for current sensing, inductor

DC resistance (DCR) sensing; Figure 14 shows a

simplified schematic for DCR method.

The current sensing method and current limit threshold

can be selected using the ILIM pin using Table 17.

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