MAX8790ETP+T Maxim Integrated Products, MAX8790ETP+T Datasheet - Page 18

• Manufacturer Part Number: MAX8790ETP+T
• Description: IC LED DRVR WHITE BCKLGT 20-TQFN
• Manufacturer: Maxim Integrated Products
• Type: Backlight, White LEDr

Specifications of MAX8790ETP+T

• Topology: PWM, Step-Up (Boost)
• Number Of Outputs: 6
• Internal Driver: No
• Type - Primary: Backlight
• Type - Secondary: White LED
• Frequency: 1MHz
• Voltage - Supply: 4.5 V ~ 5.5 V, 5.5 V ~ 26 V
• Voltage - Output: 5V
• Mounting Type: Surface Mount
• Package / Case: 20-TQFN Exposed Pad
• Operating Temperature: -40°C ~ 85°C
• Current - Output / Channel: 25mA
• Internal Switch(s): Yes
• Low Level Output Current: 1 A
• High Level Output Current: 1 A
• Operating Supply Voltage: 4.5 V to 26 V
• Maximum Power Dissipation: 1349 mW
• Maximum Operating Temperature: + 85 C
• Mounting Style: SMD/SMT
• Minimum Operating Temperature: - 40 C
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Efficiency: -
• Lead Free Status / Rohs Status: Details

where 51mV is a scale factor based on slope compen-
sation, and R
mine the minimum inductor value, the R
temporarily calculated using the following equation:
where 100mV is the current-limit sense voltage.
The minimum inductor value should be recalculated
after the R
Selection section).
Choose an available inductor value from an appropriate
inductor family. Calculate the maximum DC input cur-
rent at the minimum input voltage V
servation of energy and the expected efficiency at that
operating point ( η
in the Typical Operating Characteristics :
Calculate the ripple current at that operating point and
the peak current required for the inductor:
When DCM operating mode is chosen to minimize the
inductor value, the calculations are different from that in
the above CCM mode. The maximum inductor value for
DCM mode is calculated by the following equation:
The peak inductor current in DCM mode is calculated
using the following equation:
The inductor’s saturation current rating should exceed
I
exceed I
inductor with less than 0.1Ω series resistance.
Six-String White LED Driver with Active
Current Balancing for LCD Panel Applications
18
PEAK
I
PEAK
______________________________________________________________________________________
=
I
and the inductor’s DC current rating should
L
RIPPLE
I
DCM MAX
2
OUT
IN(DC,MAX)
×
S
f
(max)
OSC MAX
I
IN DCMAX
(
S
I
is determined (see the Sense-Resistor
PEAK
=
R
(
is the current-sense resistor. To deter-
L f
× ×
S TMP
(
V
×
2
_
IN MIN
)
,
MIN
OSC MIN
=
(
=
V
⎛
⎜
⎝
. For good efficiency, choose an
OUT MAX
)
) taken from an appropriate curve
1
I
V
IN DCMAX
(
L V
×
)
IN MIN
−
=
)
(
=
×
V
(
(
×
V
OUT MAX
1 2
)
I
OUT MAX
OUT MAX
× ×
.
(
OUT MAX
,
V
V
η
)
IN MIN
OUT MAX
)
×
×
(
2
100
(
(
(
I
(
(
IN DCMAX
V
(
V
×
V
)
OUT MAX
OUT MAX
IN MIN
(
(
η
+
mV
(
)
)
I
(
(
)
)
RIPPLE
×
× η
)
,
+
×
×
I
IN(MIN),
)
OUT MAX
2
V
V
)
f
)
−
)
MIN
OSC
DIODE
OUT
+
+
)
V
V
V
IN MIN
(
DIODE
DIODE
(
using con-
⎞
⎟ ×
⎠
S
)
)
−
)
)
V
can be
IN MIN
(
)
)
Considering the typical operating circuit, the maximum
load current (I
and a minimal input voltage of 7V. Choosing a DCM
operating mode and estimating efficiency of 90% at this
operating point:
An inductance less than L
4.7µH inductor is chosen. The peak inductor current at
minimum input voltage is calculated as follows:
The detected signal is fed into the step-up converter
control compensation loop through the CS pin.
The MAX8790’s current-mode step-up converter senses
the switch current from CS to GND with an external
resistor, R
100mV. The required resistance is calculated based
upon the peak inductor current at the end of the switch
on-time:
where 25.6mV is a scale factor from slope compensa-
tion, V
Electrical Characteristics table (85mV), and the D
the maximum duty cycle at minimum input voltage and
maximum output voltage. In DCM operating mode, it is
calculated by the following equation:
For the typical operating circuit as Figure 1:
Again, R
rent-sense resistor is chosen.
I
PEAK
CS_EC
R
=
D
2 0 825
S
R
S
MAX
×
S
<
120
S
is calculated as a maximum, so a 56mΩ cur-
L
4 7
. The current-limit sense voltage is a fixed
.
<
85
DCM MAX
.
μ ×
mA
is the current-sense voltage listed in the
V
OUT(MAX)
mV
H
=
CS EC
(
4 7
MHz
× ×
D
0 675
.
_
+
2 28 72
MAX
(
.
7
μ
25 6
V
H
)
)
×
+
.
=
2
MHz
×
Sense-Resistor Selection
=
28 72
.
1 35
) is 120mA with a 28.72V output
25 6
mV
⎛
⎜
⎝
1 35
×
.
1
L I
.
0 9
.
.
−
V
×
.
×
7
I
×
DCM(MAX)
×
mV
PEAK
A
28 72
V
A
0 9
V
LIM
V
IN MIN
(
(
.
28 72
0 75 0 68
×
.
×
(
.
×
×
.
0 75
120
×
(
(
.
7
V
28 72
0 75
f
OSC
−
V
V
)
.
+
mA
.
MHz
+
0 4
.
0 4
.
is required, so a
−
V
.
=
D
V
+
)
V
5 8
MAX
=
⎞
⎟ ×
⎠
0 4
=
−
.
.
0 68
64
7
.
V
μ
V
)
H
m
)
)
=
Ω
1 35
MAX
.
A
is