ZL2008 datasheet(21/43 Pages) RENESAS

21 / 43 page

ZL2008

FN6859 Rev 4.00

Page 21 of 43

April 29, 2011

CL (which is the average IOUTDCR product) is small and can be

neglected. Therefore, the minimum value of R1 may be

approximated Equation 26:

where PR1pkg-max is the maximum power dissipation

specification for the resistor package and P is the derating factor

for the same parameter (eg.: PR1pkg-max = 0.0625W for 0603

package, P = 50% @ 85°C). Once R1-min has been calculated,

solve for the maximum value of CL from Equation 27:

and choose the next-lowest readily available value (e.g.: 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 expressed in Equation 28:

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

For the RDS(ON) current sensing method, the external low side

MOSFET will act as the sensing element as indicated in

Figure 16.

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

Equation 30:

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 ZL2008 supports “lossless” current sensing by measuring

the voltage across a resistive element that is already present in

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 a

current sensing method. The ZL2008 incorporates two methods

for current sensing, synchronous MOSFET RDS(ON) sensing and

inductor DC resistance (DCR) sensing; Figure 16 shows a

simplified schematic for each method. The current sensing

method can be selected using the CFG2 pin, as shown in

Tables 26 and 28, or via the I2C/SMBus interface. Please refer to

Application Note AN2033 for details.

In addition to selecting the current sensing method, the ZL2008

gives the power supply designer several choices for the fault

response during over or under current condition. The user can

select the number of violations allowed before declaring fault, a

blanking time and the action taken when a fault is detected.

The blanking time represents the time when no current

measurement is taken. This is to avoid taking a reading just after

a switching transition (less accurate due to potential ringing). It is

a configurable parameter.

Once the sensing method has been selected, the user must

select the voltage threshold (VLIM), the desired current limit

threshold, and the resistance of the sensing element.

The current limit threshold voltage can be selected by simply

connecting the ILIM pin as shown in Table 14. The ground-

referenced sensing method is being used in this mode. By

default, the IOUT_CAL_GAIN is set to 1m

 for DCR mode and

 for RDS mode.

 



pkg

OUT















max

min

(EQ. 26)

DCR







min

max

(EQ. 27)

100





















avg

DCR





(EQ. 28)

5 R





(EQ. 29)

SENSE

LIM





(EQ. 30)

TABLE 14. Current Limit Threshold Voltage Pin-strap Settings

ILIM Pin

RDS VLIM

(mV)

DCR VLIM

(mV)

LOW

OPEN

HIGH

VIN

VOUT

ISENA

ISENB

Inductor DCR Sensing

(VOUT must be less than 4.0 V)

VIN

VOUT

ISENA

ISENB

MOSFET RDS(ON) Sensing

FIGURE 16. Current Sensing Methods

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