ZL6105ALAF datasheet(19/35 Pages) INTERSIL

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ZL6105

FN6906.5

December 19, 2013

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 as shown in Equation 26:

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 5k

Ω 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 IOUTDCR product) is small and can be

neglected. Therefore, the minimum value of R1 may be

approximated by Equation 27:

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 Equation 28:

Next, 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 as shown in Equation 29.

The value of R2 should be 2kΩ.

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

MOSFET will act as the sensing element as indicated in

Figure 12.

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 ZL6105 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 ZL6105 incorporates two methods

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

inductor DC resistance (DCR) sensing; Figure 12 shows a

simplified schematic for each method. The current sensing

method can be selected via the I2C/SMBus interface. Please

refer to Application Note AN2033 for details.

DCR

⋅

(EQ. 26)

() (

)

pkg

OUT

⋅

−

max

min

(EQ. 27)

DCR

⋅

−

min

max

(EQ. 28)

100

⋅

⎟

⎠

⎞

⎜

⎝

⎛

⋅

−

avg

DCR

(EQ. 29)

VIN

VOUT

ISENA

ISENB

Inductor DCR Sensing

(VOUT must be less than 4.0 V)

VIN

VOUT

ISENA

ISENB

MOSFET RDS(ON) Sensing

FIGURE 12. CURRENT SENSING METHODS

SENSE

LIM

(EQ. 30)

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