isl97652 Intersil Corporation, isl97652 Datasheet - Page 12

• Manufacturer Part Number: isl97652
• Description: 4-channel Integrated Lcd Supply With Dual Vcom Amplifiers
• Manufacturer: Intersil Corporation

The boost converter uses a summing amplifier architecture
consisting of gm stages for voltage feedback, current
feedback and slope compensation. A comparator looks at
the peak inductor current cycle by cycle and terminates the
PWM cycle if the current limit is reached.
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 60kΩ is recommended.
The boost converter output voltage is determined by the
following equation:
The current through the MOSFET is limited to 2.8Apeak.
This restricts the maximum output current (average) based
on the following equation:
Where ΔIL is peak to peak inductor ripple current, and is set by:
where f
The following table gives typical values (margins are
considered 10%, 3%, 20%, 10% and 15% on V
and I
When operating at the lower frequency option, 650kHz, the
potential increase in ripple current in the inductor can be
avoided by increasing the inductor by the same factor. This
allows the slope compensation in the boost feedback to
remain the same as the 1.3MHz case and this will maintain
stability of the converter over the widest operating range.
Operation at 650kHz allows boost operation down to lower
minimum duty cycles, where the output voltage required is
closer to the input voltage than can be achieved when the
V
I
ΔI
OMAX
BOOST
L
V
TABLE 1. MAXIMUM OUTPUT CURRENT CALCULATION
IN
=
12
12
12
12
OMAX
8
8
8
8
(V)
V
---------
s
=
L
IN
is the switching frequency
=
⎛
⎝
×
I
):
LMT
R
-------------------- -
---- -
f
D
S
1
R
+
V
2
–
O
R
15
15
18
18
15
15
18
18
ΔI
--------
2
2
(V)
L
×
⎞
⎠
V
×
FB
V
---------
V
IN
O
L (µH)
6.8
6.8
6.8
6.8
6.8
6.8
6.8
6.8
12
f
s
0.65
0.65
0.65
0.65
(MHz)
1.3
1.3
1.3
1.3
IN
I
OMAX
, V
1890
1955
1500
1590
1200
1275
1050
950
O
, L, f
(EQ. 2)
(EQ. 3)
(EQ. 4)
(mA)
S
ISL97652
higher frequency option is selected. The minimum boost
duty cycle of the ISL97652 is ~10% for 650kHz and ~20%
for 1.3MHz. When the operating duty cycle is lower than the
minimum duty cycle, the part will not switch in some cycles
randomly, which will cause some LX pulses to be skipped. In
this cas, LX pulses are not consistent any more, but the
output voltage (A
R2. Because some LX pulses are skipped, the ripple current
in the inductor will become bigger. Under the worst case, the
ripple current will be from 0 to the threshold of the current
limit. In turn, the bigger ripple current will increase the output
voltage ripple. Hence, it will need more output capacitors to
keep the output ripple at the same level. When the input
voltage equals, or is larger than, the output voltage, the
boost converter will stop switching. The boost converter is
not regulated any more, but the part will still be on and other
channels are still regulated.
Boost Converter Input Capacitor
An input capacitor is used to suppress the voltage ripple
injected into the boost converter. A ceramic capacitor with
capacitance larger than 10µF is recommended. The voltage
rating of input capacitor should be larger than the maximum
input voltage. Some capacitors are recommended in Table 2
for input capacitor.
Boost Inductor
The boost inductor is a critical part which influences the
output voltage ripple, transient response, and efficiency.
Values of 3.3µH to 10µH should be selected to match the
internal slope compensation. The inductor must be able to
handle the following average and peak current:
Some inductors are recommended in Table 3.
I
I
INDUCTOR
CAPACITOR
LAVG
LPK
4.55A
2.9A
10µF/25V
10µF/25V
3A
TABLE 2. BOOST CONVERTER INPUT CAPACITOR
6.8µH/
6.8µH/
5.2µH/
PEAK
=
PEAK
TABLE 3. BOOST INDUCTOR RECOMMENDATION
PEAK
=
I
LAVG
------------ -
1 D
I
–
O
RECOMMENDATION
DIMENSIONS
7.6X7.6X3.0 Sumida
10x10.1x3.8 Cooper
+
7.3x6.8x3.2
ΔI
--------
SIZE
1210
1210
2
(mm)
VDD
L
) is still regulated by the ratio of R1 and
TDK
Murata
VENDOR
TDK
Bussmann
VENDOR
RLF7030T-6R8N3R0
CDR7D28MNNP-6R8NC
CD1-5R2
C3225X7R1E106M
GRM32DR61E106K
PART NUMBER
PART NUMBER
November 2, 2007
FN9287.1
(EQ. 5)
(EQ. 6)