ADP1822

Rev. B | Page 15 of 24

the square-root sum of the squares of the two ripples applies, or

[]

()()

2

2

8

Δ

)

(

Δ

Δ

⎥

⎦

⎤

⎢

⎣

⎡

+

≅

SW

OUT

L

L

OUT

f

C

I

ESR

I

V

(6)

The low-side MOSFET does not carry the transition losses but

does carry the inductor current when the high-side MOSFET is

off. For high input and low output voltages, the low-side

MOSFET carries the current most of the time, and therefore to

achieve high efficiency, it is critical to optimize the low-side

MOSFET for low on resistance. In some cases, where the power

loss exceeds the MOSFET rating, or lower resistance is required

than is available in a single MOSFET, connect multiple low-side

MOSFETs in parallel. The equation for low-side MOSFET

power loss is

SELECTING THE MOSFETS

The choice of MOSFET directly affects the dc-to-dc converter

performance. The MOSFET must have low on resistance to

In addition, the MOSFET must have low thermal resistance to

ensure that the power dissipated in the MOSFET does not result

in excessive MOSFET die temperature.

() ( )

⎥

⎦

⎤

⎢

⎣

⎡

−

≅

IN

OUT

ON

LOAD

LS

V

V

R

I

P

1

2

(11)

where:

P

The high-side MOSFET carries the load current during on time

and carries all the transitions losses of the converter. Typically,

the lower the MOSFET on resistance, the higher the gate charge

and vice versa. Therefore, it is important to choose a high-side

MOSFET that balances the two losses. The conduction loss of

the high-side MOSFET is determined by

LS

is the low-side MOSFET on resistance.

R

is the total on resistance of the low-side MOSFET(s).

ON

If multiple low-side MOSFETs are used in parallel, use the

to solve this equation.

() ( )

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

≅

IN

OUT

ON

LOAD

C

V

V

R

I

P

2

(7)

SETTING THE CURRENT LIMIT

The internal current-limit circuit measures the voltage across

the low-side MOSFET to determine the load current. When the

low-side MOSFET current exceeds the current limit, the high-

side MOSFET is not allowed to turn on until the current drops

below the current limit.

where:

R

= MOSFET on resistance.

ON

The gate-charging loss is approximated by

(

)( )( )

SW

G

PVCC

T

f

Q

V

P ≅

(8)

50 μA CSL current. When the low-side MOSFET voltage is

equal to or greater than the offset voltage, the ADP1822 is in

current limit mode and prevents additional on-time cycles.

where:

Choose the current limit resistor by the equation

The high-side MOSFET transition loss is approximated by

()

2

SW

F

R

LOAD

IN

SW

f

t

t

I

V

P

×

+

×

×

=

(9)

(

)

(

)

μA

42

ONWC

LPK

CL

R

I

R

=

(12)

where:

R

where:

P

ONWC

is the worst-case (maximum) low-side MOSFET on

resistance.

SW

= high-side MOSFET switching loss power.

The total power dissipation of the high-side MOSFET is the

sum of all the previous losses, or

( ) ( ) ( )

SW

T

C

HS

P

P

P

P

+

+

≅

(10)

The worst-case, low-side MOSFET on resistance can be found

in the MOSFET data sheet. Note that MOSFETs typically

increase on resistance with increasing die temperature. To

determine the worst-case MOSFET on resistance, calculate the

worst-case MOSFET temperature (based on the MOSFET

power loss) and multiply by the ratio between the typical

on resistance at that temperature and the on resistance at 25°C

as listed in the MOSFET data sheet.

where P is the total high-side MOSFET power loss.

HS