MC33567

http://onsemi.com

6

The output capacitor capacitance and ESR required for

using the MTD3055VL as external driver are calculated as

follows:

wf +

1

Ci ·Ro

+

1

(1240 pF

) 600 pF) · (50 W) +

10.87 MHz

wp + 1

w1 )

1

wf

*1

+

1

5MHz

)

1

10.87 MHz

*1

+ 3.42 MHz

1

20 · 1

)

wa

(3 ·

wp)

· 1

gm v

Rs v

(3 ·

wp)

wa

· 1

gm

1

20 · 1

)

5MHz

(3 · 3.42 MHz)

·

1

8.8 mhos

v Rs v

(3 · 3.42 MHz)

5MHz

·

1

8.8 mhos

3.8 m

W v Rs v 233.2 mW

selecting an ESR of 30 m

W, we have:

Co w

5

Rs

·

1

wa )

1

wp

Co w

5

30 m

W

·

1

5MHz

)

1

3.42 MHz

Co w 82.07 mF

100

µF is selected as it is an industry standard value.

Please note that if the system is designed to work with

several drivers, the system has to be designed around the

driver with higher gain to insure stability for all of them.

The design guidelines discussed in this section are

conservative enough that satisfactory results may be

obtained with devices that lie just outside of these

guidelines, although deviation from these guidelines will

generally cause instability. For a more detailed analysis on

linear regulators stability please refer to ON Semiconductor

application note AND8037/D.

Adjustable Output Voltage:

The MC33567 will regulate Vout to its preset voltage level,

referenced at the sense pin. However, other Vout levels can

be obtained scaling the sense voltage. This is done using a

resistive network between the load and the sense pin as

shown in Figure 6.

Figure 6. Output Voltage Scaling Using

Resistive Network

SENSE

Vin

R2

DRV

RINT

+

Vout(new)

−

+

Vout

−

R1

ISENSE

1.8 k

W

The regulator will increase the load voltage until the

SENSE pin voltage reaches the regulator voltage level, Vout.

The new output voltage, Vout(new), is calculated as follows:

Vout(new) + Vout ) R1 *

Vout

R2

) ISENSE

Please note that in this configuration R2 and the sense

internal resistor are in parallel. The parallel combination

will reduce the effective resistance of R2. If R2 is in the range

of RINT, the parallel combination will be almost half of the

original intended value of R2. This will cause Vout(new) to be

smaller than calculated using the above equation. This is

avoided making R2 as small as possible, probably in the

range of 10 to 50 Ohms. Vout(new) is limited by the external

driver drain current and its required Gate−Source voltage as

well as the Drive Output Voltage, Vdrv.

PCB Layout Guidelines

It is recommended that the MC33567 be placed as

physically close as possible to the external series pass

MOSFET transistors. Use short traces to minimize

extraneous signals from being induced on the SENSE or

DRV line. Also, avoid routing the SENSE line near the load

and input current path, as well as the GND return current

path to prevent signal coupling.