MAX17127 Maxim Integrated Products, MAX17127 Datasheet - Page 16

• Manufacturer Part Number: MAX17127
• Description: Six-String WLED Driver
• Manufacturer: Maxim Integrated Products

The full-scale LED current is set by:
The acceptable resistance range for ISET is 120kI <
R
current of 30mA > I
The MAX17127 includes a thermal-protection circuit.
When the local IC temperature exceeds +150NC (typ),
the controller and current sources shut down. When
the thermal shutdown happens, the FPO output pin is
asserted low. The controller and current sources do not
restart until the next enable signal is sent or input supply
is recycled.
All MAX17127 designs should be prototyped and tested
prior to production.
External component value choice is primarily dictated
by the output voltage and the maximum load current, as
well as maximum and minimum input voltages. Begin by
selecting an inductor value. Once the inductor is known,
choose the diode and capacitors.
To ensure the stable operation, the MAX17127 includes
slope compensation, which sets the minimum inductor
value. In continuous conduction mode (CCM), the minimum
inductor value is calculated with the following equation:
where:
SF is a scale factor from the slope compensation
depending on input voltage (this allows a higher current
capability), the L
for stable operation in CCM, and R
the equivalent sensing scale factor from the controller’s
internal current-sense circuit.
Six-String WLED Driver with Integrated
Step-Up Converter
L
16
ISET
CCM(MIN)
_____________________________________________________________________________________
Step-Up Converter Current Calculation
< 360kI, which corresponds to full-scale LED

 


 
SF
=
=
(
SF 72mV, when V
1
V
I
LED_MAX
+
Full-Scale and Low-Level LED Current
OUT(MAX)
CCM(MIN)
=
V
72mV
IN
LED_MAX
10.6V
−
12.5V
=
2 SF f
+
is the minimum inductor value
Design Procedure
20mA 180k
×
V
, when V
DIODE
> 10mA.
R
Thermal Shutdown
×
ISET
×
IN
SW(MIN)
− ×
<
S
2 V
12.5V
IN
Ω
= 15mI (typ) is
>
IN(MIN)
12.5V
)
×
R
S
The controller can also operate in discontinuous con-
duction mode (DCM). In this mode, the inductor value
can be lower, but the peak inductor current is higher
than in CCM. In DCM, the maximum inductor value is
calculated with the following equation:
where the L
DCM, E is the nominal regulator efficiency (85%), and
I
The output current capability of the step-up regulator
is a function of current limit, input voltage, operating
frequency, and inductor value. Because the slope com-
pensation is used to stabilize the feedback loop, the
inductor current limit depends on the duty cycle, and is
determined with the following equation:
where SF is the scale factor from the slope compensa-
tion, 2.5A is the current limit specified at 75% duty cycle,
and D is the duty cycle.
The output current capability depends on the current-
limit value and operating mode. The maximum output
current in CCM is governed by the following equation:
where I
nominal regulator efficiency (85%), and D is the duty
cycle. The corresponding duty cycle for this current is:
where V
diode and R
(0.2I typ).
OUT(MAX)
I
OUT_CCM(MAX)
×
L
LIM
DIODE
DCM(MAX)
2 f

 


 
I
is the maximum output current.
×
LIM
is the current limit calculated above, E is the
DCM(MAX)
D
I
LIM
ON
SW(MAX)
=
=
V
is the forward voltage of the rectifier
is the internal MOSFET’s on-resistance
=
SF
R
OUT
S
R
SF
V
=
=
S
×
OUT







(1.27-D), when D
−
×
1
I
V
×
is the maximum inductor value for
LIM
−
IN(MIN)
I
0.97, when D 30%
LIM
V
V
OUT(MAX)
−
OUT(MAX)
−
V
×
IN
0.5 D V
R
2
f
+
ON
SW
V
× η
× ×
V
IN(MIN)
DIODE
+
×
×
V
L
+
I
DIODE
OUT(MAX)
IN
<
V
DIODE
>
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


30%
×
V
V
OUT
IN




× η