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8-String WLED Driver with Integrated Step-Up
Regulator and SMBus/PWM Dimming Capability
I OUT(MAX) O 2 O V OUT(MAX) O ( V OUT(MAX) + V DIODE - V S(MIN) )
(
)
I PEAK =
120mA O 2 O 32V O ( 32V + 0.4V - 7V )
3.3μH O 1.1MHz O 0.85 O ( 32V + 0.4V )
The peak inductor current in DCM is calculated with the
following equation:
L O f SW(MIN) O EO V OUT(MAX) + V DIODE
The inductor’s saturation current rating should exceed
I PEAK , and the inductor’s DC current rating should
exceed I IN(DC,MAX) . For good efficiency, choose an
inductor with less than 0.1 I series resistance.
Considering the circuit with eight 10-LED strings and
I PEAK = = 1.40A
Output Capacitor Selection
The total output voltage ripple has two components: the
capacitive ripple caused by the charging and discharg-
ing on the output capacitor, and the ohmic ripple due to
the capacitor’s equivalent series resistance (ESR):
V RIPPLE = V RIPPLE(C) + V RIPPLE(ESR)
I OUT(MAX) ? V OUT(MAX) - V S(MIN) ?
? V OUT(MAX) SW ?
20mA LED full-scale current, the maximum load current
(I OUT(MAX) ) is 160mA with a 32V output and a minimal
input voltage of 7V.
Choosing a CCM operating mode with LIR = 0.7 at 1MHz
and an estimating efficiency of 85% at this operating point:
and:
V RIPPLE(C) ≈
C OUT ? O f ?
? ?
? 7V ? ?
? ? 0.85 ?
32V - 7V
32V ? ? 160mA × 1MHz ? ? ? ?
L = ? ? ? ? ? = 9.08μH
2
? 0.7
In CCM, the inductor has to be higher than L CCM(MIN) :
L CCM(MIN) = ( 32V + 0.4V - 2 O 7V ) O 13.7m I = 5.5μH
2 O 25.5mV O 0.9MHz
10 F H inductor is chosen, which is higher the minimum L
that guarantees stability in CCM.
The peak inductor current at minimum input voltage is
calculated as follows:
V RIPPLE(ESR) ≈ I PEAK R ESR(COUT)
where I PEAK is the peak inductor current (see the
Inductor Selection section).
The output voltage ripple should be low enough for the
FB_ current source regulation. The ripple voltage should
be less than 200mV P-P . For ceramic capacitors, the out-
put voltage ripple is typically dominated by V RIPPLE(C) .
The voltage rating and temperature characteristics of the
output capacitor must also be considered.
Rectifier Diode Selection
The MAX17105’s high switching frequency demands a
160mA O 32V 7V O ( 32V - 7V )
I PEAK =
+
7V O 0.85 2 O 10μH O 32V O 0.9MHz
= 1.16A
high-speed rectifier. Schottky diodes are recommended
for most applications because of their fast recovery time
and low forward voltage. The diode should be rated to
L DCM(MAX) = ? 1-
(7V) 2 O 0.85
Alternatively, choosing a DCM operating mode will
lower inductance and estimate efficiency of 85% at
this operating point. Since DCM will have higher peak
inductor current at lower input, it will cause current limit
when the parameters are not chosen properly. Consider
the case with six 10-LED strings and 20mA LED full-
scale current to prevent excessive switch current from
causing current limit:
? 7V ?
?
? 32V + 0.4V ?
O = 3.9μH
2 O 1.1MHz O 32V O 120mA
3.3 F H inductor is chosen. The peak inductor current at
minimum input voltage is calculated as follows:
handle the output voltage and the peak switch current.
Make sure that the diode’s peak current rating is at least
I PEAK calculated in the Inductor Selection section and
that its breakdown voltage exceeds the output voltage.
Overvoltage-Protection Determination
The overvoltage-protection circuit should ensure the
circuit safe operation; therefore, the controller should
limit the output voltage within the ratings of all MOSFET,
diode, and output capacitor components, while provid-
ing sufficient output voltage for LED current regulation.
The OVP pin is connected to the center tap of a resistive
voltage-divider (R1 and R2 in Figure 1) from the high-
voltage output. When the controller detects the OVP pin
voltage reaching the threshold V OVP_TH , typically 1.25V,
overvoltage protection is activated. Hence, the step-up
converter output overvoltage protection point is:
_______________________________________________________________________________________
23
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