Texas Instruments UCC2977EVM - Manual (Page 16)

Push-Pull Inductor Selection
2-2
2.1 Push-Pull Inductor Selection
The push-pull topology requires a different approach for calculating the
external inductor values. Referring to Figure 1–1, Q2A and Q2B are driven out
of phase at 50% duty cycle. When Q2A is on, current is ramped up in L1.
During the next switch cycle Q2B is turned on, Q2A is turned off, and the
energy stored in L1 is transferred through the piezoelectric transformer. L1
resonates with the piezoelectric primary capacitance forming a half sinusoid
at Q2A’s drain. In order to achieve zero-voltage switching, the drain voltage
must return to ground before the next switching cycle. This dictates that the
LC resonant frequency must be greater than the switching frequency.
L
t 1
4 p 2 ƒ 2 Cp
(1)
In the EVM with a 100 kHz switching frequency and 61.6 nF piezoelectric
transformer, L1 and L2 must be less than 41
µ
H for zero-voltage switching, so
22-
µ
H inductors are used.
A suitable capacitor must be chosen so that the peak-to-peak output ripple is
within the limits allowable for the application.
2.2 Frequency Range Setting
The VCO frequency range is programmed with external components R1, R3
and C2. The frequency is determined by the voltage decay from 1.7 V to 0.6 V
at the OSC pin. When the voltage reaches 0.6 V, an internal current source
charges OSC back to 1.7 V. The decay time is determined by the value of C2
and the discharge currents generated in R1 and R3. An accurate NPO
capacitor is recommended for C2 and 1% resistors are recommended for R1
and R3. The output frequency range can be calculated by equation 2:
ƪ
R1 ) R3
R1 R3 C2
ƫ
(2)
2 ln
ȧ
ȱ
Ȳ
1.7 V (R1
)
R3)
*
V
COMP
R1
ǒ
0.6 V
*
V
COMP
Ǔ
R1
)
0.6 V R3
ȧ
ȳ
ȴ
ƒ
ǒ
V
COMP
Ǔ
+
With C2 set at 220 pF, R1 at 22 k
Ω
and R3 at 182 k
Ω
, the frequency is 100 kHz.
2.3 Analog Dimming of the Lamp
For analog dimming, enable the open-lamp detection by connecting pin 1 to
pin 2 of jumper JP1 and ground BD. Lamp intensity is controlled with the signal
AD.
The RMS lamp current becomes:
I
LAMP
+
ƪ
1.5 V (R2
)
R10)
*
V
AD
R10
ƫ
p
2
Ǹ
R2 R15
(3)