REV. A

ADP3158/ADP3178

–12–

Trade-Offs Between DC Load Regulation and AC Load

Regulation

Casual observation of the circuit operation—e.g., with a voltmeter

—would make it appear that the dc load regulation appears to

be rather poor compared to a conventional regulator (see Figure

4). This would be especially noticeable under very light or very

heavy loads where the voltage is “positioned” near one of the

extremes of the regulation window rather than near the nominal

center value. It must be noted and understood that this low gain

characteristic (i.e., loose dc load regulation) is inherently required

to allow improved transient containment (i.e., to achieve tighter

ac load regulation). That is, the dc load regulation is intentionally

sacrificed (but kept within specification) in order to minimize

the number of capacitors required to contain the load transients

produced by the CPU.

68pF

2.5V

ADP3158/

ADP3178

1k

250m

1 F

3.3V

100 F

2.5V, 2.2A

LRDRV1

LRFB1

10k

Figure 6. Adding Overcurrent Protection to the

Linear Regulator

Linear Regulators

The two linear regulators provide a low cost, convenient, and

versatile solution for generating additional supply rails. The

maximum output load current is determined by the size and

thermal impedance of the external N-channel power MOSFET

that is placed in series with the supply. The output voltage is

sensed at the LRFB pin and compared to an internal reference

voltage in a negative feedback loop which keeps the output voltage

in regulation. If the load is reduced or increased, the MOSFET

drive will also be reduced or increased by the controller IC to

provide a well-regulated

± 2.5% accurate output voltage.

The LRFB threshold of the ADP3158 are internally set at 2.5 V

(LRFB1) and 1.8 V (LRFB2), while the LRFB pins of the

ADP3178 are compared to an internal 1 V reference. This allows

the use of an external resistor divider network to program the linear

regulator output voltage. The correct resistor values for setting the

output voltage of the linear regulators in the ADP3178 can be

determined using:

VV

RR

R

OUT LR

LRFB

UL

L

+

(32)

R

kV

V

V

R

kV

V

V

k

U

OUT LR

LRFB

LRFB

U

=

×−

()

=

×−

()

=

10

10

1 2

1

1

2

Ω

Ω

Ω

()

.

(33)

Efficiency of the Linear Regulators

The efficiency and corresponding power dissipation of each

of the linear regulators are not determined by the controller

IC. Rather, these are a function of input and output voltage and

load current. Efficiency is approximated by the formula:

η=

×

100%

V

V

OUT

IN

(34)

The corresponding power dissipation in the MOSFET, together

with any resistance added in series from input to output, is given by:

PV

V

I

LDO

IN

OUT

OUT

=×

(

–

)

(35)

Minimum power dissipation and maximum efficiency are accom-

plished by choosing the lowest available input voltage that exceeds

the desired output voltage. However, if the chosen input source

is itself generated by a linear regulator, its power dissipation will

be increased in proportion to the additional current it must

now provide.

Implementing Current Limit for the Linear Regulators

The circuit of Figure 4 gives an example of a current limit pro-

tection circuit that can be used in conjunction with the linear

regulators. The output voltage is internally set by the LRFB

pin. The value of the current sense resistor may be calculated

as follows:

R

mV

I

mV

A

m

S

O MAX

≅==

Ω

540

540

22

250

()

.

(36)

The power rating of the current sense resistor must be at least:

PR

I

W

DRS

SO MAX

()

(

)

.

=×

=

2

12

(37)

The maximum linear regulator MOSFET junction temperature

with a shorted output is:

TT

V

I

TC

C W

V

A

C

MAX

A

C

IN

O MAX

MAX

JJ

J

()

()

()

()

(.

/

( .

.

)

=+

×

×

=° +

°

×

×

=°

θ

50

14

33

22

60

(38)

which is within the maximum allowed by the MOSFET’s data

sheet specification. The maximum MOSFET junction tempera-

ture at nominal output is:

TT

V

V

I

TC

C W

V

V

A

C

NOM

A

C

IN

OUT

O NOM

NOM

JJ

J

()

()

()

((

–

))

(.

/

( .

– .)

)

=+

×

×

=° +

°

×

×

=

°

θ

50

14

33

25

2

52

(39)

This example assumes an infinite heatsink. The practical limita-

tion will be based on the actual heatsink used.