EL7585 INTERSIL [Intersil Corporation], EL7585 Datasheet - Page 11

• Manufacturer Part Number: EL7585
• Description: TFT-LCD Power Supply
• Manufacturer: INTERSIL [Intersil Corporation]

Cascaded MOSFET Application
A 20V N-channel MOSFET is integrated in the boost
regulator. For the applications where the output voltage is
greater than 20V, an external cascaded MOSFET is needed
as shown in Figure 22. The voltage rating of the external
MOSFET should be greater than V
Linear-Regulator Controllers (V
V
The EL7585 includes three independent linear-regulator
controllers, in which two are positive output voltage (V
and V
V
applications circuits are shown in Figures 23, 24, and 25
respectively.
Calculation of the Linear Regulator Base-Emitter
Resistors (R
For the pass transistor of the linear regulator, low frequency
gain (Hfe) and unity gain freq. (f
datasheet. The pass transistor adds a pole to the loop
transfer function at f
phase margin at low frequency, the best choice for a pass
device is often a high frequency low gain switching
transistor. Further improvement can be obtained by adding a
base-emitter resistor R
Block Diagram), which increase the pole frequency to:
f
lowest value R
enough base current (I
current (I
We will take as an example the V
Fairchild FMMT549 PNP transistor is used as the external
pass transistor, Q5 in the application diagram, then for a
maximum V
sheet indicates Hfe_min = 100.
p
LOGIC
V
FIGURE 22. CASCADED MOSFET TOPOLOGY FOR HIGH
OFF
=f
IN
T
*(1+ Hfe *re/R
LOGIC
)
linear-regulator controller functional diagrams,
C
).
LOGIC
), and one is negative. The V
OUTPUT VOLTAGE APPLICATIONS
BE
BL
EL7585
, R
in the design as long as there is still
BE
operating requirement of 500mA the data
p
=f
BP
)/Hfe, where re=KT/qIc. So choose the
B
T
BE
) to support the maximum output
/Hfe. Therefore, in order to maintain
and R
(R
LX
BP
11
T
, R
BN
) are usually specified in the
LOGIC
BL
BOOST
)
, R
FB
ON
BN
linear regulator. If a
, V
ON
.
in the Functional
LOGIC
, V
OFF
, and
, and
V
BOOST
ON
EL7585
The base-emitter saturation voltage is: Vbe_max = 1.25V
(note this is normally a Vbe ~ 0.7V, however, for the Q5
transistor an internal Darlington arrangement is used to
increase it's current gain, giving a 'base-emitter' voltage of
2 x V
(Note that using a high current Darlington PNP transistor for
Q5 requires that V
voltage be required, then an ordinary high gain PNP
transistor should be selected for Q5 so as to allow a lower
collector-emitter saturation voltage).
For the EL7585, the minimum drive current is:
I_DRVL_min = 8mA
The minimum base-emitter resistor, R
calculated as:
R
1.25V/(8mA - 500mA/100) = 417Ω
This is the minimum value that can be used - so, we now
choose a convenient value greater than this minimum value;
say 500Ω. Larger values may be used to reduce quiescent
current, however, regulation may be adversely affected, by
supply noise if R
0.9V
PG_LDOP
BL
_min = V
BE
FIGURE 23. V
).
+
GMP
-
+
BE
-
_max/(I_DRVL_min - Ic/Hfe_min) =
BL
1: Np
IN
is made too high in value.
ON
CLAMP
LDO_ON
> V
ESD
36V
FUNCTIONAL BLOCK DIAGRAM
LOGIC
DRVP
FBP
+ 2V. Should a lower input
R
7kΩ
BP
BL
20kΩ
R
R
, can now be
V
P1
P2
BOOST
Q3
CP (TO 36V)
V
ON
0.1µF
(TO 35V)
0.1µF
C
July 1, 2005
ON
FN7345.1
LX