Rainbow Electronics MAX17127 - Manual (Page 16)

16 _____________________________________________________________________________________
MAX17127
Six-String WLED Driver with Integrated
Step-Up Converter
Full-Scale and Low-Level LED Current
The full-scale LED current is set by:
LED_MAX
ISET
20mA 180k
I
R
× Ω
=
The acceptable resistance range for ISET is 120kI <
R
ISET < 360kI, which corresponds to full-scale LED
current of 30mA > I LED_MAX > 10mA.
Thermal Shutdown
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.
Design Procedure
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.
Step-Up Converter Current Calculation
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:
( )
OUT(MAX) DIODE IN(MIN) S
CCM(MIN)
SW(MIN)
V V 2 V R
L
2 SF f
+ − Ч Ч
=
Ч Ч
where:
IN
IN
IN
SF 72mV, when V 12.5V
72mV
SF , when V 12.5V
V 12.5V
1
10.6V
= < 

= > 
−

+

SF is a scale factor from the slope compensation
depending on input voltage (this allows a higher current
capability), the L
CCM(MIN) is the minimum inductor value
for stable operation in CCM, and R S = 15mI (typ) is
the equivalent sensing scale factor from the controller’s
internal current-sense circuit.
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:
IN(MIN)
DCM(MAX)
OUT(MAX) DIODE
2
IN(MIN)
SW(MAX) OUT(MAX) OUT(MAX)
V
L 1
V V
V
2 f V I
 
= −  
  +
 
× η
Ч
Ч Ч Ч
where the L DCM(MAX) is the maximum inductor value for
DCM, E is the nominal regulator efficiency (85%), and
I
OUT(MAX) is the maximum output current.
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:
LIM
S
LIM
S
SF
I 0.97, when D 30%
R
SF
I (1.27-D), when D 30%
R

= × <



= × >

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:
IN IN
OUT_CCM(MAX) LIM
SW OUT
0.5 D V V
I I
f L V
  Ч Ч
= − Ч Ч η  
×
 
where I LIM is the current limit calculated above, E is the
nominal regulator efficiency (85%), and D is the duty
cycle. The corresponding duty cycle for this current is:
OUT IN DIODE
OUT LIM ON DIODE
V V V
D
V I R V
− +
=
− × +
where V DIODE is the forward voltage of the rectifier
diode and R ON is the internal MOSFET’s on-resistance
(0.2I typ).