6

ML4828

SETTING THE OSCILLATOR FREQUENCY

The ML4828 switching frequency is determined by the

charge and discharge times of the network connected to

between the internal clock and the charge and discharge

times.

RAMP PEAK

RAMP VALLEY

2.5V

1.25V

INTERNAL

CLOCK

Figure 3. Internal Oscillator Timing.

The frequency of the oscillator is:

f

tt

OSC

CHARGE

DISCHARGE

=

+

1

(1)

The ramp peak is 2.5V and the ramp valley is 1.25V,

giving a ramp range of 1.25V. The charging current is set

I

V

R

CHARGE

T

= 25

.

(2)

while the discharging current is fixed at 1.4 mA. The

charge and discharge times can be determined by:

t

CV

I

CR

CHARGE

T

CHARGE

TT

=

×

=

×

125

2

.

(3)

t

CV

I

CV

mA

DISCHARGE

T

DISCHARGE

T

=

×

=

×

125

1 25

14

..

.

(4)

The oscillator frequency can then be found by substituting

the results of equations 3 and 4 into equation 1. This

frequency activates a T flip-flop which generates the

output pulses. The T flip-flop acts as a frequency divider

(

÷2), so the output frequency will be:

f

f

OUT

OSC

=

2

(5)

ERROR AMPLIFIER

The ML4828 error amplifier has a 10MHz bandwidth and

a 10V/

µs slew rate. Figure 4 gives the Bode plot of the

error amplifier.

100

80

60

40

20

0

–20

180

135

90

45

0

100

1K

10K

100K

1M

10M

100M

FREQUENCY

GAIN

PHASE

Figure 4. Error Amplifier Open-Loop Gain

and Phase vs. Frequency.

OUTPUT DRIVERS

The ML4828 has four high-current CMOS output drivers,

each capable of 1A peak output current. These outputs

have been designed to quickly switch the gates of power

MOSFET transistors via a gate drive transformer. For higher

power applications, the outputs can be connected to

external MOSFET drivers.

The output phase delay times are set by charging an

internal 6.7pF capacitor up to the REF voltage (2.5V) via a

for the side A and side B drivers, respectively. The

charging current and delay time for side A are given by:

I

V

R

A

A

= 25

.

(6)

tpF R

DA

A

=×

67

.

(7)

tpF

k

ns

Ω =

6 7

33

220

.

(8)