4

UCC1837

UCC2837

UCC3837

of the CAP voltage should the external load exceed the

average current, which occurs during load and line tran-

sient conditions. The value of CAP also determines the

startup time of the linear regulator. The voltage at CAP

charges up with a time constant determined by the

charge pump output impedance (typically 80k

Ω) and the

value of the capacitor on CAP.

An external voltage such as +12V may be tied to the

CAP pin directly to insure a higher value of VOUT, which

may be useful when a standard level MOSFET is used or

when VDD is very low and the resulting VOUT voltage

may need to be higher. With an external source applied

to CAP, the maximum voltage at VOUT will be approxi-

mately 1V below the external source.The external +12V

source should be decoupled to GND using a minimum of

a 0.01

µF capacitor.

Choosing a Pass Element

The UCC3837 is designed for use with an N-channel

MOSFET pass element only. The designer may choose

a logic level or standard gate level MOSFET depending

on the input voltage, the required gate drive, and the

available voltage at VOUT as discussed previously.

MOSFET selection should be based on required dropout

MOSFET is used when low dropout is required, but this

type of MOSFET will have higher gate capacitance which

may result in a slower transient response.

A MOSFET used in linear regulation is typically operated

at a gate voltage between the threshold voltage and the

gate plateau voltage in order to maintain high gain. This

mode of operation is linear, and therefore the channel re-

sistance is higher than the manufacturer’s published

the non-linear (switch) mode under transient conditions,

when minimum dropout voltage is required.

Disabling the UCC3837

Grounding the CAP pin will remove the drive voltage and

effectively disable the output voltage. The device used to

short the output of CAP should have a very low leakage

current when in the OPEN state, since even a few

microamps will lower the charge pump voltage.

A second method of disabling the UCC3837 is to place a

of a quicker restart time as the voltage at CAP will not be

completely discharged. The charge pump will be loaded

down by the typical 40

µA charging current of the error

amplifier with this configuration, resulting in a lower volt-

age at CAP.

Compensating the Error Amplifier

Using a MOSFET as an external pass element intro-

duces a pole in the control loop that is a function of the

the MOSFET input gate capacitance. Fig. 3 indicates

that in the normal operation of a linear regulator using a

MOSFET, the gate capacitance can be predicted directly

from the MOSFET characteristic charge curve, using the

relationship:

C

Qgth

Vgth

∆

∆

This pole can be canceled by programming a zero fre-

quency on the output of the UCC3837 error amplifier

equal to the pole frequency. Therefore:

8

1VDD

CS

5

VOUT

2CAP

7CT

4COMP

6

FB

3

GND

Q1

IRL2203N

OR EQUIVALENT

R2

1.8k

R3

1.5k

C3

1000

µF

3.3V

R1

0.020

R

COMP

10k

C

COMP

820pF

0.1

µF

0.1

µF

Q1

C1

330

µF

5V

ON/OFF

UCC3837

Figure 1. Typical application 5V to 3.3V, 5A

APPLICATION INFORMATION

5

6

7

8

9

10

11

12

13

14

15

34

56

78

9

10

11

12

VDD

E/A DISABLED

LINEAR REGULATOR

OVERDRIVE

UDG-99137