Rainbow Electronics MAX781 - Manual (Page 18)

MAX781
VPPA and VPPB are linear regulated from VHI, so the
power consumed by loads on VPPA or VPPB equals
the VHI voltage times the load current. Barring a
Schottky diode drop, VHI equals the 3OUT voltage x (1
+ turns ratio). With the specifications of Table 7 and
an 80% transformer efficiency, the total power con-
sumption works out to 2.037W; thus the average cur-
rent through the primary of T1 is 594mA. The peak
current through the primary of T1 will depend on the
minimum primary inductance. As a rule of thumb, the
peak current will be about 1.5 times the average cur-
rent. For an average current of 594mA, the peak cur-
rent would be about 900mA. To achieve a 900mA peak
current, select R7 to be 80mV / 900mA = 0.088
Ω
. The
closest comercially available value would be 0.082
Ω
±1%. The spreadsheet in Listing 1 calculates how high
a guaranteed output current can be, given commercial-
ly available component values, and taking component
tolerances into account.
Table 8 shows the electrical specifications for a trans-
former that meets the requirements of Listing 1.
Power MOSFETs M1 and M2 should have an on-resis-
tance at logic-level gate drive (r
DS(ON) at V GS = 4.5V)
of the same order as T1’s primary DC resistance. The
Siliconix Si9955DY dual N-channel MOSFET satisfies
this requirement with a 0.2
Ω
maximum on-resistance
per device.
Table 9 lists the bill of materials for an example circuit
that fulfills the requirements of Table 7.
Driving a CCFL Backlight Royer Oscillator
The digitally adjustable current from the battery charger
can be used to drive a Royer oscillator. The Royer
oscillator is a resonant circuit fed by a constant current.
The root mean square current out of the secondary
winding of the Royer transformer is proportional to the
current fed into the center tap of the Royer transformer.
Figure 10 shows the application circuit. The diode from
VCHG to BATT keeps BATT from dropping too far
below 5OUT, which causes excess supply current.
Figure 11 shows how the programmed current corre-
sponds to the CCFL root mean square tube current.
The NPN transistor connected to COMP and the zener
diode protects the transformer from an open-tube con-
dition by shutting off the Si9953DY if pin 2 of the
CTX110606 exceeds 0.6V + 10V + 0.6V. This limits the
voltage on the secondary to 11.2 x 171 x 2 = 3830.4V
peak-to-peak = 1354V
RMS , which is well within the
CTX110606 maximum secondary voltage specification
of 2010V
RMS .
PDA/Hand-Held Computer Power Controller
18 ______________________________________________________________________________________
Table 7. Specifications for a Typical Design Table 8. Dale Electronics M/N LPE-6562-A070
Specifications
DESIGNATION
5 NiCD cells
Input Power
15V DC adapter
PARAMETER
3.3V Output
Current
MIN
Charge Current
Digitally
programmable
5
VPPA Output
Current
+12V output when
programming flash
memory in Slot A
300
0
60
UNITS
V
mA
A
mA
MAX
17.5
1
VPPB Output
Current
+12V output when
programming flash
memory in Slot A
60 mA
TYP
Primary Inductance 20
PARAMETER
Primary Continuous
DC Current
2.6
Secondary DC Resistance
Primary DC Resistance
MAX
24
0.51
0.075
A
UNITS
µH
Ω
Ω
MIN
16
Leakage Inductance
(at 0.1V
RMS , 100kHz)
µH 0.03
Turns Ratio
(secondary/primary)
3.5