10
FN7317.2
February 4, 2005
The following table gives typical values:
(Margins are considered 10%, 3%, 20%, 10%, and 15% on
V
IN
, V
O
, L, F
S
, and I
LMT
, respectively)
Component Considerations
Input Capacitor
It is recommended that C
IN
is larger than 10µF.
Theoretically, the input capacitor has ripple current of ΔI
L
.
Due to high-frequency noise in the circuit, the input current
ripple may exceed the theoretical value. Larger capacitor will
reduce the ripple further.
Boost Inductor
The inductor has peak and average current decided by:
The inductor should be chosen to be able to handle this
current. Furthermore, due to the fixed internal
compensation, it is recommended that maximum inductance
of 10µH and 15µH to be used in the 5V and 12V or higher
output voltage, respectively.
The output diode has average current of I
O
, and peak current
the same as the inductor's peak current. Schottky diode is
recommended and it should be able to handle those currents.
Feedback Resistor Network
An external resistor divider is required to divide the output
voltage down to the nominal reference voltage. Current
drawn by the resistor network should be limited to maintain
the overall converter efficiency. The maximum value of the
resistor network is limited by the feedback input bias current
and the potential for noise being coupled into the feedback
pin. A resistor network in the order of 200kΩ is
recommended. The boost converter output voltage is
determined by the following relationship:
where V
FBB
is 1.300V.
Schottky Diode
Speed, forward voltage drop, and reverse current are the
three most critical specifications for selecting the Schottky
diode. The entire output current flows through the diode, so
the diode average current is the same as the average load
current and the peak current is the same as the inductor
peak current. When selecting the diode, one must consider
the forward voltage drop at the peak diode current. On the
Elantec demo board, MBRM120 is selected. Its forward
voltage drop is 450mV at 1A forward current.
Output Capacitor
The EL7584 is specially compensated to be stable with
capacitors which have a worst-case minimum value of 10µF
at the particular V
OUT
being set. Output ripple voltage
requirements also determine the minimum value and the
type of capacitors. Output ripple voltage consists of two
components - the voltage drop caused by the switching
current though the ESR of the output capacitor and the
charging and discharging of the output capacitor:
For low ESR ceramic capacitors, the output ripple is
dominated by the charging/discharging of the output
capacitor.
In addition to the voltage rating, the output capacitor should
also be able to handle the RMS current is given by:
Positive and Negative Charge Pump (V
ON
and
V
OFF
)
The EL7584 contains two independent charge pumps (see
charge pump block and connection diagram.) The negative
charge pump inverts the V
DDN
supply voltage and provides
a regulated negative output voltage. The positive charge
pump doubles the V
DDP
supply voltage and provides a
regulated positive output voltage. The regulation of both the
negative and positive charge pumps is generated by the
internal comparator that senses the output voltage and
compares it with and internal reference. The switching
frequency of the charge pump is set to ½ the boost converter
switching frequency.
The pumps use pulse width modulation to adjust the pump
period, depending on the load present. The pumps are short-
circuit protected to 180mA at 12V supply and can provide
15mA to 60mA for 6V to 12V supply.
TABLE 1. MAXIMUM CONTINUOUS OUTPUT CURRENT
V
IN
(V) V
O
(V) L (ΜH) F
S
(kHz) I
OMAX
(mA)
3.3 9 10 1000 430
3.3 12 10 1000 320
3.3 15 10 1000 250
5 9 10 1000 650
5 12 10 1000 470
5 15 10 1000 370
12 18 10 1000 830
I
LPK
I
LAVG
ΔI
L
2
--------
+=
I
LAVG
I
O
1 - D
-------------
=
V
BOOST
R
1
R
2
+
R
1
---------------------
V
FBB
×=
V
RIPPLE
I
LPK
ESR
V
OUT
- V
IN
V
OUT
--------------------------------
+×
I
OUT
C
OUT
FS×
------------------------------
×=
I
CORMS
1( - D) D
ΔI
L
2
I
LAVG
2
--------------------
+
⎝
⎜
⎜
⎛
×
1
12
------
×
⎠
⎟
⎟
⎞
I
LAVG
×=
EL7584
