el7520a Intersil Corporation, el7520a Datasheet - Page 12

• Manufacturer Part Number: el7520a
• Description: 4-channel Dc/dc Controller
• Manufacturer: Intersil Corporation

Input Capacitor
The input capacitor is used to supply the current to the
converter. It is recommended that C
The reflected ripple voltage will be smaller with larger C
The voltage rating of input capacitor should be larger than
maximum input voltage.
Boost Inductor
A 3.3µH inductor is recommended due to the fixed internal
slope compensation. The inductor must be able to handle
the following average and peak current:
I
I
Switching MOSFET
Due to the parasitic inductance of the trace, the MOSFET
will experience spikes higher that the output voltage when
the MOSFET turns off. Thus, a MOSFET with enough
voltage margin is needed.
The R
and current limit. A MOSFET with low R
get high efficiency and output current, but very low R
will reduce the loop stability. A MOSFET with 20mΩ to 50mΩ
R
are shown in following table.
Rectifier Diode
A high-speed diode is desired due to the high switching
frequency. Schottky diodes are recommended because of
their fast recovery time and low forward voltage. The rectifier
diode must meet the output current and peak inductor
current requirements.
Output Capacitor
The output capacitor supplies the load directly and reduces the
ripple voltage at the output. Output ripple voltage consists of
two components: the voltage drop due to the inductor ripple
current flowing through the ESR of output capacitor, and the
charging and discharging of the output capacitor.
FDC655AN
FDS4488
Si7844DP
SI6928DQ
LAVG
LPK
DS(ON)
NUMBER
PART
=
DS(ON)
V
=
I
RIPPLE
LAVG
------------ -
1 D
is recommended. Some recommended MOSFETs
I
–
O
TABLE 2. RECOMMENDED MOSFETs
MANUFACTURER
+
of the MOSFET is critical for power dissipation
∆I
--------
Semiconductor
Semiconductor
=
2
L
I
Fairchild
Fairchild
LPK
Vishay
Vishay
×
ESR
12
+
V
----------------------- -
6.3A, 30V, R
7.9A, 30V, R
10A, 30V, R
20A, 30V, R
O
V
–
O
V
IN
IN
be larger than 10µF.
×
DS(ON)
FEATURE
--------------- -
C
OUT
I
DS(ON)
DS(ON)
DS(ON)
DS(ON)
O
×
is desired to
---- -
f
1
S
= 22mΩ
= 30mΩ
= 23mΩ
= 22mΩ
EL7520, EL7520A
DS(ON)
IN
.
For low ESR ceramic capacitors, the output ripple is
dominated by the charging and discharging of the output
capacitor. The voltage rating of the output capacitor should
be greater than the maximum output voltage.
Compensation
The EL7520 and EL7520A can operate in either P mode or
PI mode. Connecting C
P mode. For better load regulation, use PI mode with a
2.2nF capacitor between C
Linear-Regulator Controllers (V
V
The EL7520, EL7520A include three independent linear-
regulator controllers, in which two are positive output voltage
(V
and V
applications circuits are shown in Figures 27, 28, and 29
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
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
increase the pole frequency to: f
where re = KT/qIc. So choose the lowest value R
design as long as there is still enough base current (I
support the maximum output current (I
We will take as an example the V
Fairchild FMMT549 PNP transistor is used as the external
pass transistor, Q31 in the application diagram, then for a
maximum V
sheet indicates Hfe_min = 100.
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 EL7520, EL7520A, the minimum drive current is:
I_DRVL_min = 8mA
OFF
BE
ON
(R
and V
LOGIC
)
BP
, R
p
LOGIC
LOGIC
BL
= f
linear-regulator controller functional diagrams,
, R
BL
T
/Hfe. Therefore, in order to maintain phase
BN
, R
), and one is negative. The V
IN
operating requirement of 500mA the data
in the Functional Block Diagram), which
BP
> V
INT
LOGIC
and R
INT
pin directly to V
T
p
and ground.
) are usually specified in the
+ 2V. Should a lower input
BN
= f
LOGIC
T
)
*(1+ Hfe *re/R
C
ON
).
linear regulator. If a
, V
BE
IN
LOGIC
).
will enable
BE
ON
BE
, V
in the
B
July 12, 2005
, and
) to
)/Hfe,
OFF
FN7318.0
,