ZL6105ALAF datasheet(18/35 Pages) INTERSIL

18 / 35 page

ZL6105

FN6906.5

December 19, 2013

Without capacitive filtering near the power supply circuit, this

current would flow through the supply bus and return planes,

coupling noise into other system circuitry. The input capacitors

should be rated at 1.2x the ripple current calculated in Equation

13 to avoid overheating of the capacitors due to the high ripple

current, which can cause premature failure. Ceramic capacitors

with X7R or X5R dielectric with low ESR and 1.1x the maximum

expected input voltage are recommended.

BOOTSTRAP CAPACITOR SELECTION

The high-side driver boost circuit utilizes an external Schottky diode

(DB) and an external bootstrap capacitor (CB) to supply sufficient

gate drive for the high-side MOSFET driver. DB should be a 20mA,

30V Schottky diode or equivalent device and CB should be a 1µF

ceramic type rated for at least 6.3V.

QL SELECTION

The bottom MOSFET should be selected primarily based on the

device’s rDS(ON) and secondarily based on its gate charge. To

choose QL, use Equation 14 and allow 2% to 5% of the output

power to be dissipated in the rDS(ON) of QL (lower output voltages

and higher step-down ratios will be closer to 5%):

Calculate the RMS current in QL as shown in Equation 15:

Calculate the desired maximum rDS(ON) as shown in Equation 16:

Note that the rDS(ON) given in the manufacturer’s datasheet is

measured at +25°C. The actual rDS(ON) in the end-use application

will be much higher. For example, a Vishay Si7114 MOSFET with

a junction temperature of +125°C has an rDS(ON) that is 1.4x

higher than the value at +25°C. Select a candidate MOSFET, and

calculate the required gate drive current as shown in Equation

17:

Keep in mind that the total allowed gate drive current for both QH

and QL is 80mA.

MOSFETs with lower rDS(ON) tend to have higher gate charge

requirements, which increases the current and resulting power

required to turn them on and off. Since the MOSFET gate drive

circuits are integrated in the ZL6105, this power is dissipated in

the ZL6105 according to the Equation 18:

QH SELECTION

In addition to the rDS(ON) loss and gate charge loss, QH also has

switching loss. The procedure to select QH is similar to the

procedure for QL. First, assign 2% to 5% of the output power to

be dissipated in the rDS(ON) of QH using Equation 18 for QL. As

was done with QL, calculate the RMS current as shown in

Equation 19:

Calculate a starting rDS(ON) as shown in Equation 20. This

equation uses 5% as an example:

Select a MOSFET and calculate the resulting gate drive current.

Verify that the combined gate drive current from QL and QH does

not exceed 80mA.

Next, calculate the switching time using Equation 22:

where Qg is the gate charge of the selected QH and Igdr is the

peak gate drive current available from the ZL6105.

Although the ZL6105 has a typical gate drive current of 3A, use

the minimum guaranteed current of 2A for a conservative

design. Using the calculated switching time, calculate the

switching power loss in QH using Equation 23:

The total power dissipated by QH is given by Equation 24:

MOSFET THERMAL CHECK

Once the power dissipations for QH and QL have been calculated,

the MOSFETs junction temperature can be estimated. Using the

junction-to-case thermal resistance (Rth) given in the MOSFET

manufacturer’s datasheet and the expected maximum printed

circuit board temperature, calculate the junction temperature as

shown in Equation 25:

CURRENT SENSING COMPONENTS

Once the current sense method has been selected (Refer to

“Current Limit Threshold Selection” on page 19”), the

components are selected as indicated in the following.

When using the inductor DCR sensing method, the user must

also select an R/C network comprised of R1 and CL (see

Figure 11).

OUT

= 05

(EQ. 14)

Lrms

botrms

−

(EQ. 15)

()2

)

(

botrms

(EQ. 16)

(EQ. 17)

INM

(EQ. 18)

Lrms

toprms

(EQ. 19)

OUT

= 05

(EQ. 20)

()2

)

(

toprms

(EQ. 21)

gdr

(EQ. 22)

OUT

INM

swtop

(EQ. 23)

swtop

QHtot

(EQ. 24)

()

pcb

max

(EQ. 25)

ISENA

ISENB

FIGURE 11. DCR CURRENT SENSING

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