IRU3065 datasheet(6/15 Pages) IRF

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IRU3065(PbF)

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in figure 1. The schottky diode can be replaced with a

100Ω resistor (Figure 28.) with a small sacrifice of effi-

ciency but lower cost.

Thermal Consideration

The thermal design is to ensure maximum junction tem-

perature of IRU3065 will not exceed the maximum op-

eration junction temperature, which is 125 C. The junc-

tion temperature can be estimated by the following:

TJ = PD×ΘJA+TA≤TJ(MAX) = 125 C

Where ΘJA is the thermal resistance from junction to

case which is usually provided in the specification. PD

is the power dissipation. TA is the ambient temperature.

The package thermal resistance of IRU3065 is estimated

as 230C/W due to compact package.

Assuming the maximum allowed ambient temperature

is 70C, the maximum power dissipation of IRU3065 will

PD<(125-70C)/ΘJA=(125-70)/230=240mW

For High Power Application

The IR3065 driver is designed to driver PMOS for low

current applications. Figure 4. shows the rise time ver-

sus cap load. For big capacitor load, the rise time is

increasing.

Rise time versus cap load

00.511.522.5

Cap(nF)

Fig.4. Rise time versus cap load.

The internal gate driver of IRU3065 is designed for

load current up to 1A. For higher power applications,

external driver is recommended to driver the external

FETs.

Demo board Evaluation Results

Fig.1 shows the evaluation board schematic and the

selected components. The diode D1 can be replaced

with a 100ohm resistor. The measured efficiency ver-

sus load current is shown in Fig. 6. With the boot strap

schottky diode, the efficiency is slight higher compar-

ing with using 100ohm resistor.

If higher efficiency is preferred, lower operation fre-

quency should be selected. Figure. 5 shows a effi-

ciency curve when 4.7uH inductor is chosen. The maxi-

mum operation frequency reduces from 800k to 250kHz.

As a results, efficiency is more than 10% higher.

For the application circuit shown in Fig.1. The mea-

sured output voltage versus output current is shown in

Figure 7. When the load current approaches 400mA,

the output voltage starts to drop and goes into power

limit mode. When output is about 1A, the output voltage

will goes almost zero.

The measured frequency versus load is listed in Fig-

ure 8. The highest switching frequency occurs at about

440mA. As load current goes up, the IC goes into

power limit mode and frequency automatically goes down

to protect the system.

The current sensing comparator threshold voltage ver-

sus VCC is shown in Figure. 9. Since this threshold is

only a divided voltage of VCC, it will changes when

VCC changes. This should be aware in the application.

The output voltage versus Vin=VCC is shown in fig-

ure 11. Since the voltage reference is set by Vin. When

Vin changes, the output voltage will change along Vin.

Sometimes this feature is preferrable since Vout may

want to be tracked with Vin except the polarity. How-

ever, if more accurate output is required, a external

voltage reference should set the output voltage.

For the evaluation board, the measured inductor volt-

age waveforms are listed in Figure 13-17. Figure 15

shows the measured inductor voltage waveform when

output current is 250mA, which the converter is oper-

ated in regulation mode and output voltage is regulated

at desired voltage -5V. Figure 16 shows the measured

inductor voltage waveform when the output current is

equal to the critical current IOCP. Figure 17 shows the

measured inductor voltage waveform when the output is

in short circuit, which indicates that the converter is in

power limit mode and output voltage is near zero.

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