LTC1704 Linear Technology, LTC1704 Datasheet - Page 25

• Manufacturer Part Number: LTC1704
• Description: 550kHz Synchronous Switching Regulator Controller Plus Linear Regulator Controller
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
where V
V
out voltage.
The MJD44H11 from ON Semiconductor has a V
around 0.9V at I
is 1.1V maximum with 30mA of drive current.
If the computed minimum V
requirement of 3.15V then 3.15V should be used.
The minimum V
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
A typical 1.5V V
mum V
V
If a V
be 3.15V and the minimum V
External NPN Pass Transistor
The external NPN Pass transistor for the LTC1704 linear
regulator supply should be selected based on the follow-
ing criteria:
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
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
regulator can follow an output load change without losing
voltage regulation.
DROPOUT
INREG
1. Maximum output current
2. DC current gain h
3. Total allowable power dissipation
4. Gain bandwidth product f
OUTREG
CC
of 1.5V + 0.2V = 1.7V to operate.
BE(QEXT)
T
is the LTC1704 linear regulator controller drop-
of 1.5V + 0.9V + 1.1V = 3.5V and a minimum
of the transistor determines how fast the linear
of 0.8V is needed, the minimum V
C
INREG
OUTREG
= 2A, 25 C and the LTC1704’s V
is base emitter voltage of QEXT and
U
is determined by the V
FE
, 2A application will need a mini-
U
CC
INREG
T
is less than the LTC1704
FE
must be large enough
is 0.8V + 0.2V = 1V.
W
C
CE
= 2A, 25 C.
saturation
U
CC
DROPOUT
should
BE
of
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
power dissipation rating is 1.75W without heat sink and
the gain bandwidth product f
cally 50MHz.
Linear Regulator Supply Current Limit Programming
The LTC1704 linear regulator uses an external resistor
R
This indirectly programs the linear regulator current limit
threshold. Figure 13 shows the setup. One end of the
resistor R
source V
V
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 A 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
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
I
where V
(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
LT
CC
REGILM
REGILM
:
R
pin. The other end of the resistor is connected to the
REGILM
REGON
to program the NPN pass transistor base current.
REGON
and the linear regulator current limit threshold
REGILM
is the pull-up voltage source for R
or, alternatively, it can be connected to the
V
FE
REGON
LTC1704/LTC1704B
is connected to an external voltage
, which makes this current limit scheme
h
I
LT
FE
– .
– .
7 5
0 8 2100
mA
T
of the MJD44H11 is typi-
FE
is 60 at IC = 2A. The
25
REGILM
1704bfa