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°C and the LTC1704’s 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°C.
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 1.9µA current source, programs the REGDR maxi-
mum output current. The actual linear regulator current
limit depends on the pass transistor’s 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
=
()()
–.
–.
0 8 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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