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8-String WLED Driver with Integrated Step-Up
Regulator and SMBus/PWM Dimming Capability
25.5mV O ( 0.75 - D )
I LIM = 2A +
0.5 O D O V S ?
V S
I OUT_CCM(MAX) = ? I LIM - ? O OE
(
)
L CCM(MIN) =
D =
V S(MIN)
L DCM(MAX) = ? 1- ?
V OUT(MAX) DIODE ? ?
?
2 O f SW(MAX) OUT(MAX) OUT(MAX)
O V
O I
2 O V OUT O ( V OUT DIODE S )
I OUT_DCM(MAX) =
Thermal Shutdown
The MAX17105 includes a thermal-protection circuit.
When the local IC temperature exceeds +150 N C (typ),
the controller and current sources shut down and do not
restart until the next enable signal is sent.
Design Procedure
All MAX17105 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 stable operation, the MAX17105 includes
slope compensation, which sets the minimum induc-
tor value. In continuous-conduction mode (CCM), the
minimum inductor value is calculated with the following
equation:
V OUT(MAX) + V DIODE - 2 O V S(MIN) O R S
2 O 25.5mV O f SW(MIN)
where 25.5mV is a scale factor from the slope com-
pensation, L CCM(MIN) is the minimum inductor value
for stable operation in CCM, and R S =13.7m I (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:
? ?
? + V
V S(MIN) 2 OE
O
where 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 compensation is
used to stabilize the feedback loop, the inductor current
limit depends on the duty cycle, and is determined with
the following equation:
R S
where 25.5mV is the scale factor from the slope compen-
sation, 2A 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:
?
? f SW O L ? V OUT
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:
V OUT -V S + V DIODE
V OUT -I LIM O R ON + V DIODE
where V DIODE is the forward voltage of the rectifier diode
and R ON is the internal MOSFET’s on-resistance (0.15 I
(typ)).
The maximum output current in DCM is governed by the
following equation:
L O I LIM 2 O f SW O E O ( V OUT + V DIODE )
+ V -V
Inductor Selection
The inductance, peak current rating, series resistance,
and physical size should all be considered when select-
ing an inductor. These factors affect the converter’s oper-
ating mode, efficiency, maximum output load capability,
transient-response time, output voltage ripple, and cost.
_______________________________________________________________________________________
21
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