Lamp Output Frequency
(External f
EL
Signal)
When the f
EL
pin is driven by an external clock, the
lamp frequency is f
EL
/4. The EV kit uses an ICM7556
dual CMOS timer (U2) to generate a 50% duty-cycle
square wave. The VR3 value can be adjusted while
power is applied (see Table 6).
Boost Converter Frequency
(R
SLEW
, C
SW
)
ICM7556 square-wave frequency f
EL
= 1/(1.4 x R x C).
The boost converter switching frequency can be set by
an external capacitor, C
SW
(see Table 7).
Boost Converter Frequency
(External f
SW
Signal)
Boost converter switching frequency can be driven by
an external clock. The EV kit uses an ICM7556 dual
CMOS timer (U2) to generate a 90% duty-cycle square-
wave pulse. The C
SW
value can be adjusted while
power is applied (see Table 8).
f
RC
SW
SLEW SW
=
×
361.
Evaluates: MAX4990
MAX4990 Evaluation Kit
_______________________________________________________________________________________ 5
ICM7556
T IM IN G C A PAC IT OR ( p F )
VR3
TRIMMER
ICM7556 TIMING
RESISTORS (kΩ)
f
EL
FROM ICM7556
(kHz)
LAMP FREQUENCY =
f
EL
/4 (Hz)
0% CW VR3 + R5 = 513.3 0.0927 23
50% CW VR3 + R5 = 263.3 0.180 45
C7 = 15000
100% CW VR3 + R5 = 13.3 3.58 895
Table 6. Lamp Output Frequency When JU4 = Pins 1-2 (External f
EL
)
CW = Clockwise. Approximate trimmer values provided for initial guidance only.
C
SW
(pF) JU1 SHUNT POSITION VR1 TRIMMER
R
SLEW
(MΩ)
f
SW
(kHz)
1-2 — R2 = 0.374 142
25% CW VR1 + R1 = 0.560 95
C3 = 68
2-3
50% CW VR1 + R1 = 0.997 53
Table 7. Boost Converter Frequency When JU5 = Open (Internal f
SW
)
CW = Clockwise. Approximate trimmer values provided for initial guidance only.
VC1 TRIMMER
ICM7556
TIMING CAPACITOR (pF)
f
SW
= ICM7556 SQUARE WAVE
= 1.44/((R7 + 2 x R6) x (VC1)) (kHz)
Minimum: 0° VC1 = 7 + 25 112
Center: 90° or 270° VC1 = 30 + 25 60
Maximum: 180° (50 ≤ VC1 ≤ 100) + 25 33
Table 8. Boost Converter Frequency When JU5 = Pins 1-2 (External f
SW
)
Note: f
SW
square wave has fixed duty cycle = (R6 + R7)/(R7 + 2 x R6) = 90%. Approximate trimmer values provided for initial guid-
ance only.
