ADP3153

–10–

REV. 0

The compensating capacitance is determined from the equality

of the pole frequency of the error amplifier gain and the zero

frequency of the impedance of the output capacitor.

C

RC

R

mmF

k

nF

COMP

E

OUT

COMP

==

×

=

516 2

31

26

.

.

Ω

In the application circuit we tested, we found that the compen-

sation scheme shown in Figure 2 gave the optimal response to

meet the Pentium II dc/dc static and transient specifications

with sufficient margins including the ADP3153’s initial error

tolerance, the PCB layout trace resistances, and the external

component parasitics. If we increase the load resistance to the

COMP pin, the static regulation will improve. The load transient

response, however, will get worse. In Figure 2, if we decrease the

R1 = 150 k

Ω resistor vs. the R2 = 39 kΩ resistor, the regulation

band will shift positive in relation to the 2.8 V. If we increase

the R1 resistor, the regulation band will shift negative. It may be

necessary to adjust these resistor values to obtain the best static

and dynamic regulation compliance depending on the output

capacitor ESR and the parasitic trace resistances of the PCB

layout. A detailed design procedure and published conference

papers on the optimal compensation are available on ADI’s

website (http://www.analog.com).

ADP3153 Linear Regulator

The ADP3153 linear regulator provides a low cost, convenient,

and versatile solution for generating an additional power supply

rail that can be programmed between 1.2 V–5 V. The maximum

output load current is determined by the size and thermal imped-

ance of an external N-channel power MOSFET that is placed in

series with the 5 V supply and controlled by the ADP3153. The

ADP3153 and compared to an internal 1.2 V reference in a

negative feedback loop which keeps the output voltage in regula-

tion. Thus, if the load is being reduced or increased, the FET

drive will also be reduced or increased by the ADP3153 to pro-

vide a well regulated

±1% accurate output voltage. This accu-

racy is maintained even if the load changes at the very high rate

typical of CPU-type loads. The output voltage is programmed

Figure 14.

Features

• Typical Efficiency: 66% at 3.3 V Output Voltage

• Tight DC Regulation Due to 1% Reference and High Gain

• Output Voltage Stays Within Specified Limits at Load Cur-

rent Step with 30 A/

µs Slope

• Fast Response to Input Voltage or Load Current Transients

with a maximum load current of 0.5 A. Additionally, overcurrent

protection is provided by the addition of an external NPN tran-

procedure is given below.

Linear Regulator Design Specifications

°C

Chosen FET . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . IRX3803

Junction-to-Ambient Thermal Impedance (FET)*

θ

JA

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

°C/W

*Uses 1 inch square PCB cu-foil as heatsink

as follows:

R

V

kk

k

PROG

O

=









×=









×=

2

12

120

33

12

120

35

.

–

.

.

–

ΩΩ

Ω

The current sense resistor may be calculated as follows:

R

I

S

O MAX

2

2

06

06

05

12

==

=

..

.

.

Ω

The power rating is:

PR

I

W

S

S

O MAX

22

2

2

11

036

=×

×

..

Use a 0.5

Ω resistor.

The maximum FET junction temperature at shorted output is:

TT

V

I

C

FETMAX

AMB

A

IN

O MAX

=+

×

×

×

=

+× ×

×

=

°

θ

J

2

11

50 40 5 0 5 1 1 160

.

..

which is within the maximum allowed by the FET's data sheet.

The maximum FET junction temperature at nominal output is:

TT

V

V

I

C

FETMAX

AMB

A

IN

O

O MAX

=+

×

+×

θ

J

–

–.

.

22

50 40

5 3 3

0 5 84

The output filter capacitor maximum allowed ESR is:

Ω.

This requirement is met using a 1000

µF/10 V LXV series ca-

pacitor from United Chemicon. For applications requiring

higher output current, a heatsink and/or a larger MOSFET

should be used to reduce the MOSFET’s junction to ambient

thermal impedance.

1.1

470pF

2N2222

IRX3803

2k

1000 F/10V

VLDO

ADP3153

FB

20k

35k

Figure 14. Linear Regulator with Overcurrent Protection