TK6591x
THEORY OF OPERATION (CONT.)
With properly selected components, the TK6591x will
nominally support peak output voltages to 90 V
(180 VPK-PK). Should the EL Panel become disconnected
from the driver outputs, the removal of the load can cause
the output voltage to increase beyond 90 V. To protect
against this fault condition, a clamp circuit exists on the
high voltage output which nominally limits the output
voltage to a typical value of 105 V (210 VPK-PK).
HV
HVP
-
HVP
UL
UR
+
EL
EL
EL Panel
LL LR
Current Source 2
Current Source 1
DETAILS CONCERNING THE
H-BRIDGE SECTION OPERATION
In an effort to extend EL lamp life, reduce EMI emissions,
and reduce the power draw of the IC, current sources to
control the charging and discharging of the EL lamp panel
and special sequencing control of the H-bridge FETs were
added to the H-bridge of TK659xx.
FIGURE 2: H-BRIDGE SCHEMATIC
BOTH OFF
Current sources were added between ground and the
sources of the low-side N-channel FETs (Figure 2).
Therefore, the current into and out of the EL panel is
controlled and limited.
UL
OFF
OFF
OFF
OFF
OFF
ON
UR
LL
OFF
ON
ON
BOTH ON
OFF
ON
ON
ON
ON
The FETs are turned off and on in the sequence shown in
Figure 3. As is noted in Figure 3, there is a period of time
when both of lower N-channel FETs are turned on and both
of upper P-channel FETs are turned off. This provides a
period of time to discharge the EL panel capacitance
completely;beforestartingtorechargeitagainwith current
from HV voltage rail. Therefore, this special sequencing
method prevents taking current off the HV voltage rail
duringthedischargeofELpanelcapacitanceandoperates
more efficiently.
LR
ON
OFF
-
V
V
EL
+
EL
Discharging
EL Panel
Capacitance
+
-
V
= V
V
-
EL
EL
EL
FIGURE 3: H-BRIDGE SEQUENCING WAVEFORMS
Page 10
May 2000 TOKO, Inc.
TOKO [ TOKO, INC ]
