IRF7457 datasheet(7/21 Pages) IRF

7 / 21 page

IRU3037 / IRU3037A

Rev. 2.8

03/10/03

www.irf.com

For this design, IRF7301 is a good choice. The device

provides low on-resistance in a compact SOIC 8-Pin

package.

The IRF7301 has the following data:

The total conduction losses will be:

The switching loss is more difficult to calculate, even

though the switching transition is well understood. The

reason is the effect of the parasitic components and

switching times during the switching procedures such

as turn-on / turnoff delays and rise and fall times. With a

linear approximation, the total switching loss can be ex-

pressed as:

The switching time waveform is shown in figure 4.

Figure 4 - Switching time waveforms.

From IRF7301 data sheet we obtain:

These values are taken under a certain condition test.

For more detail please refer to the IRF7301 data sheet.

By using equation (6), we can calculate the switching

losses.

Feedback Compensation

The IRU3037 is a voltage mode controller; the control

loop is a single voltage feedback path including error

amplifier and error comparator. To achieve fast transient

response and accurate output regulation, a compensa-

tion circuit is necessary. The goal of the compensation

network is to provide a closed loop transfer function with

the highest 0dB crossing frequency and adequate phase

margin (greater than 45

8).

The output LC filter introduces a double pole, –40dB/

decade gain slope above its corner resonant frequency,

and a total phase lag of 180

8 (see Figure 5). The Reso-

nant frequency of the LC filter expressed as follows:

Figure 5 shows gain and phase of the LC filter. Since we

already have 180

8 phase shift just from the output filter,

the system risks being unstable.

Figure 5 - Gain and phase of LC filter.

The IRU3037’s error amplifier is a differential-input

transconductance amplifier. The output is available for

DC gain control or AC phase compensation.

The E/A can be compensated with or without the use of

local feedback. When operated without local feedback

the transconductance properties of the E/A become evi-

dent and can be used to cancel one of the output filter

poles. This will be accomplished with a series RC circuit

from Comp pin to ground as shown in Figure 6.

VDSS = 20V

ID = 5.2A

RDS(ON) = 0.05

Where:

VDS(OFF) = Drain to Source Voltage at off time

tr = Rise Time

tf = Fall Time

T = Switching Period

ILOAD = Load Current

tr = 42ns

tf = 51ns

PSW = 0.186W

PCON(TOTAL)=PCON(Upper Switch)+PCON(Lower Switch)

PCON(TOTAL) = ILOAD

3 RDS(ON) 3 q

q = 1.5 according to the IRF7301 data sheet for

150

8C junction temperature

PCON(TOTAL) = 1.2W

FLC =

---(7)

3CO

PSW =

ILOAD

---(6)

tr + tf

VDS(OFF)

VDS

VGS

10%

90%

td(ON)

td(OFF)

Gain

FLC

0dB

Phase

FLC

-180

Frequency

-40dB/decade

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