LTC1704EGN#TRPBF【PDF 25/28 页】IC REG DL BUCK/LINEAR 16SSOP

LTC1704/LTC1704B

25

1704bfa

where V

BE(QEXT)

is base emitter voltage of QEXT and

V

DROPOUT

is the LTC1704 linear regulator controller drop-

out voltage.

The MJD44H11 from ON Semiconductor has a V

BE

of

around 0.9V at I

C

= 2A, 25癈 and the LTC1704s V

DROPOUT

is 1.1V maximum with 30mA of drive current.

If the computed minimum V

CC

is less than the LTC1704

requirement of 3.15V then 3.15V should be used.

The minimum V

INREG

is determined by the V

CE

saturation

voltage of QEXT when it is driven with a base current equal

to the maximum REGDR pin drive current. The D44H11

has a saturation voltage of around 0.2V at I

C

= 2A, 25癈.

A typical 1.5V V

OUTREG

, 2A application will need a mini-

mum V

CC

of 1.5V + 0.9V + 1.1V = 3.5V and a minimum

V

INREG

of 1.5V + 0.2V = 1.7V to operate.

If a V

OUTREG

of 0.8V is needed, the minimum V

CC

should

be 3.15V and the minimum V

INREG

is 0.8V + 0.2V = 1V.

External NPN Pass Transistor

The external NPN Pass transistor for the LTC1704 linear

regulator supply should be selected based on the follow-

ing criteria:

1. Maximum output current

2. DC current gain h

FE

3. Total allowable power dissipation

4. Gain bandwidth product f

T

The NPN transistor must be able to supply the maximum

operating current for the linear regulator supply. At the

same time, the DC current gain h

FE

must be large enough

such that the pass transistor can supply the maximum

load current with 30mA of base current. The transistor

must not be subjected to power dissipation higher than

the rated value, both during normal operation and over-

load conditions. Heat sink can be used to increase the

alloweable power dissipation rating. The gain bandwidth

product f

T

of the transistor determines how fast the linear

regulator can follow an output load change without losing

voltage regulation.

The MJD44H11 from ON Semiconductor and SGS-

Thomson can be used in the LTC1704 linear regulator

supply with current ratings up to 2A. The MJD44H11 from

ON Semiconductor can supply 8A of output current and

the minimum DC Current Gain h

FE

is 60 at IC = 2A. The

power dissipation rating is 1.75W without heat sink and

the gain bandwidth product f

T

of the MJD44H11 is typi-

cally 50MHz.

Linear Regulator Supply Current Limit Programming

The LTC1704 linear regulator uses an external resistor

R

REGILM

to program the NPN pass transistor base current.

This indirectly programs the linear regulator current limit

threshold. Figure 13 shows the setup. One end of the

resistor R

REGILM

is connected to an external voltage

source V

REGON

or, alternatively, it can be connected to the

V

CC

pin. The other end of the resistor is connected to the

REGILM pin. REGILM is internally regulated to 0.8V. The

voltage difference across this resistor generates the

REGILM pin input current. This current, together with the

internal 1.9礎 current source, programs the REGDR maxi-

mum output current. The actual linear regulator current

limit depends on the pass transistors widely distributed

DC current gain h

FE

, which makes this current limit scheme

not particularly accurate. Nevertheless, this method re-

moves the expensive current sense resistor and with

careful design, it is sufficient to protect the external NPN

from over damaging.

The following equation shows the relationship between

R

REGILM

and the linear regulator current limit threshold

I

LT

:

R

V

I

h

mA

REGILM

REGON

LT

FE

=

(

)( )

?/DIV>

.

08 2100

75

where V

REGON

is the pull-up voltage source for R

REGILM

(see Figure 13).

When there is an overload at the linear regulator output,

the current limit circuit fires and the output voltage drops.

To protect the NPN from excessive heating, the controller

APPLICATIO S I FOR ATIO

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