ZL2004 datasheet(21/42 Pages) RENESAS

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ZL2004

FN6846 Rev. 3.00

Page 21 of 42

February 15, 2011

5.7.6 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 the following

equation and allow 2–5% 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%):

OUT



 05

Calculate the RMS current in QL as follows:

Lrms

botrms







Calculate the desired maximum RDS(ON) as follows:



2

)

(

botrms



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 1.4 times

higher than the value at 25°C. Select a candidate

MOSFET, and calculate the required gate drive current

as follows:





Keep in mind that the total allowed gate drive current

for both QH and QL is 80 mA.

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

ZL1505, this power is dissipated in the ZL1505

according to the following equation:

INM





5.7.7 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 2–5% of

the output power to be dissipated in the RDS(ON) of QH

using the equation for QL above. As was done with

QL, calculate the RMS current as follows:

Lrms

toprms





Calculate a starting RDS(ON) as follows, in this example

using 5%:

OUT



 05



2

)

(

toprms



Select a MOSFET and calculate the resulting gate

drive current. Verify that the combined gate drive

current from QL and QH does not exceed 80 mA.

Next, calculate the switching time using:

gdr



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

is the peak gate drive current available from the

ZL1505.

Although the ZL1505 has a typical gate drive current

of 3.2 A, use the minimum guaranteed current of 2 A

for a conservative design. Using the calculated

switching time, calculate the switching power loss in

QH using:

OUT

INM

swtop





The total power dissipated by QH is given by the

following equation:

swtop

QHtot





5.7.8 MOSFET Thermal Check

Once the power dissipations for QH and QL have been

calculated, the MOSFET’s 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 follows:





pcb







max

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