ADP3310

–6–

REV. A

APPLICATION INFORMATION

The ADP3310 is very easy to use. A P-channel power MOSFET

and a small capacitor on the output is all that is needed to form

an inexpensive ultralow dropout regulator. The advantage of

using the ADP3310 controller is that it can drive a pass PMOS

FET to provide a regulated output at high current.

FET Selection

The type and size of the pass transistor are determined by the

threshold voltage, input-output voltage differential and load

current. The selected PMOS must satisfy the physical and

thermal design requirements. Table I shows a partial list of

manufacturers providing the PMOS devices. To ensure that the

at worst case conditions (i.e., temperature and manufacturing

× R

S

~ 0.5 V (Hot)

~ 0.9 V (Cold)

× 11 mΩ = 4.27 V

Equation (1) applies to a gate-to-source voltage less than the

gate to source clamp voltage.

≤ 5 V, logic level FET should be considered.

voltage drop due to the load current and the ON resistance of

the FET. As a safety margin, it is recommended to use a MOS-

value from Equation 1. Also, in the event the circuit is shorted

to ground, the MOSFET must be able to conduct the maximum

short circuit current. The selected MOSFET must satisfy these

criteria; otherwise, a different pass device should be used. If the

FET data is not available in the catalogue, contact the FET

manufacturer.

Thermal Design

The maximum allowable thermal resistance between the FET

junction and the highest ambient temperature must be taken

into account to determine the type of FET package used. One

square inch of PCB copper area as heatsink yields a typical

θ

°C/W for the SOT-223 package and θ

°C/W for

the SO-8 package. For substantially lower thermal resistances,

D

For normal applications, the FET can be directly mounted to the

PCB. But, for higher power applications, an external heat sink is

required to satisfy the

θ

θ

= Junction Temperature

= Maximum Output Current

θ

JA

=

125

−50

1.7

× 3

= 14.7°C/W

For such a low

θ

NDP6020P in a heatsink mountable TO-220 package, is

required. The required external heatsink is determined as

follows:

θ

CA

=

θ

θ

JC

θ

CA

= Case-to-Ambient Thermal Resistance

θ

JA

= Junction-to-Ambient Thermal Resistance

θ

JC

= Junction-to-Case Thermal Resistance

θ

JC

= 2

°C/W for NDP6020P

θ

CA

= 14.7

°C/W – 2°C/W = 12.7°C/W

For a safety margin, select a heatsink with a

θ

the value calculated above to allow extended duration of short

circuit. In a natural convection environment, a large heatsink

such as 3" length of Type 63020 extrusion from Aavid Engineering

is required.

External Capacitors

The ADP3310 is stable with virtually any good quality capaci-

tors (anyCAP™), independent of the capacitor’s minimum ESR

(Effective Series Resistance) value. The actual value of the ca-

pacitance of the external PMOS device. A 10

µF capacitor at the

output is sufficient to ensure stability for up to 10 A output

current. Larger capacitors can be used if high output current

surges are anticipated. Extremely low ESR capacitors (ESR

≈0)

such as multilayer ceramic or OSCON are preferred because

they offer lower ripple on the output. For less demanding

requirements, a standard tantalum or even an aluminum

electrolytic is adequate. However, if an aluminum electrolytic is

used, be sure it meets the temperature requirements because

aluminum electrolytic has poor performance over temperature.

Shutdown Mode

Applying a TTL high signal to the EN pin or tying it to the

input pin will enable the output. Pulling this pin low or tying it

to ground will disable the output. In shutdown mode, the

controller’s quiescent current is reduced to less than 1

µA.

Gate-to-Source Clamp

An 8 V gate-to-source voltage clamp is provided to protect the

MOSFET in the event the output is suddenly shorted to

ground. This allows the use of the new, low on-state resistance

Short Circuit Protection

The power FET is protected during short circuit conditions

with a foldback type of current limiting which significantly re-

duces the current.

Current Sense Resistor

Current limit is achieved by setting an appropriate current sense

0.05

(1.5

× I

anyCAP is a trademark of Analog Devices, Inc.