REV. 0

ADP3170

–8–

CT Selection for Operating Frequency

determined by an external timing capacitor CT. Each time the

high-side N-channel MOSFET switch turns on, the voltage

across CT is reset to approximately 0 V. During the off-time,

CT is charged by a constant current of 150

µA. Once CT reaches

3.0 V, a new on-time cycle is initiated. The value of the off-time is

calculated using the continuous-mode operating frequency.

at an output voltage of 1.8 V, the corresponding off-time is:

t

V

Vf

V

VkHz

s

OFF

OUT

IN

NOM

=









×=









×=

1

1

1

18

5

1

200

32

–

–

.

.

µ

(1)

The timing capacitor cab be calculated from the equation:

C

tI

V

sA

V

pF

T

OFF

CT

TTH

=

×

=

×

≈

()

.

3 2

150

3

150

µµ

(2)

The converter operates at the nominal operating frequency only

due to the parasitic voltage drops across the power devices. The

183 kHz (see Equation 3), where:

(estimated value: 6 m

Ω)

(estimated value: 6 m

Ω)

(estimated value: 2.5 m

Ω)

(estimated value: 3 m

Ω)

Table I. Output Voltage vs. VID Code

VID3

VID2

VID1

VID0

VID25

0

1

0

0

0

1.050 V

0

1

0

0

1

1.075 V

0

0

1

1

0

1.100 V

0

0

1

1

1

1.125 V

0

0

1

0

0

1.150 V

0

0

1

0

1

1.175 V

0

0

0

1

0

1.200 V

0

0

0

1

1

1.225 V

0

0

0

0

0

1.250 V

0

0

0

0

1

1.275 V

1

1

1

1

0

1.300 V

1

1

1

1

1

1.325 V

1

1

1

0

0

1.350 V

1

1

1

0

1

1.375 V

1

1

0

1

0

1.400 V

1

1

0

1

1

1.425 V

1

1

0

0

0

1.450 V

1

1

0

0

1

1.475 V

1

0

1

1

0

1.500 V

1

0

1

1

1

1.525 V

1

0

1

0

0

1.550 V

1

0

1

0

1

1.575 V

1

0

0

1

0

1.600 V

1

0

0

1

1

1.625 V

1

0

0

0

0

1.650 V

1

0

0

0

1

1.675 V

0

1

1

1

0

1.700 V

0

1

1

1

1

1.725 V

0

1

1

0

0

1.750 V

0

1

1

0

1

1.775 V

0

1

0

1

0

1.800 V

0

1

0

1

1

1.825 V

f

t

VI

R

R

R

V

VI

R

R

R

R

s

VA

m

m

V

V

A

m

mmm

MIN

OFF

IN

O MAX

DS ON HSF

SENSE

L

OUT

IN

O MAX

DS ON HSF

SENSE

L

DS ON LSF

=×

×+

+

()

×+

+

()

=

×

×+

×+

+

1

1

33

523

6

3

1 8

523

6

2 5

3

6

––

––

.

–(

) – .

–(

.

–

()

(

)

(

)

()

()

µ

ΩΩ

Ω

ΩΩΩ))

= 183 kHz

(3)

Inductance Selection

The choice of inductance determines the ripple current in the

inductor. Less inductance leads to more ripple current, which

increases the output ripple voltage and the conduction losses in

the MOSFETs, but allows using smaller-size inductors and, for

a specified peak-to-peak transient deviation, output capacitors

with less total capacitance. Conversely, a higher inductance

means lower ripple current and reduced conduction losses, but

requires larger-size inductors and more output capacitance for

the same peak-to-peak transient deviation. The following equa-

tion shows the relationship between the inductance, oscillator

frequency, peak-to-peak ripple current in an inductor and input

and output voltages:

For 6 A peak-to-peak ripple current, which corresponds to

approximately 25% of the 23 A full-load dc current in an inductor,

Equation 4 yields an inductance of:

L

Vs

A

nH

=

×

=

18

33

6

990

..

µ

A 1

µH inductor can be used, which gives a calculated ripple

current of 5.9 A at no load. The inductor should not saturate at

the peak current of 26 A and should be able to handle the sum

of the power dissipation caused by the average current of 23 A

in the winding and the core loss.

L

Vt

I

OUT

OFF

L RIPPLE

=

×

()

(4)