LTC1470/LTC1471
APPLICATIONS INFORMATION
winding is rectified by diode D2, added to the main 5V output
and applied to the input of the LT1313. (Note that the
auxiliary winding must be phased properly as shown in
Figure 3.)
When the 12V output is activated by a TTL high on either VPP
enable lines, the 5V section of the LTC1142HV is forced into
continuous mode operation. A resistor divider composed of
R2, R3 and switch Q3 forces an offset which is subtracted
from the internal offset at the Sense
–
input (pin 14) of the
LTC1142HV. When this external offset cancels the built-in
25mV offset, Burst Mode
TM
operation is inhibited and the
LTC1142HV is forced into continuous mode operation. (See
LTC1142HV data sheet for further detail.) In this mode, the
15V auxiliary supply can be loaded without regard to the
loading on the 5V output of the LTC1142HV.
Continuous mode operation is only invoked when the LT1313
is programmed to 12V. If the LT1313 is programmed to 0V,
3.3V or 5V, power is obtained directly from the main power
source (battery pack) through diode D1. Again, the LT1313
output can be loaded without regard to the loading of the
main 5V output.
V
IN
5.4V TO 11V
V
IN
24
PDRIVE
23
Q1
1/2 LTC1142
(3.3V REG)
NDRIVE
6
Q2
1
SENSE
+
C2
1000pF
SENSE
–
28
R3
12k
HC86
AENVPP0
AENVPP1
HC86
BENVPP0
BENVPP1
Q3
2N7002
Figure 4. Deriving 15V from the 3.3V Output of the LTC1142 for VPP Power
U
W
U
U
R4 and C4 absorb transient voltage spikes associated with
the leakage inductance inherent in T1’s secondary winding
and ensure that the auxiliary supply does not exceed 20V.
Auxiliary Power from the LTC1142 3.3V Output
For low-battery count applications (< 6.5V) it is necessary to
modify the circuit of Figure 3. As the input voltage falls, the
5V duty cycle increases to the point where there is simply not
enough time to transfer energy from the 5V primary winding
to the auxiliary winding. For applications where 12V load
currents exist in conjunction with these low input voltages,
use the circuit shown in Figure 4. In this circuit, the auxiliary
15V supply is generated from an overwinding on the 3.3V
inductor of the LTC1142 regulator output.
In Figure 3, power is drawn directly from the batteries
through D1 when the regulator is in Burst Mode operation
and the VPP pins require 3.3V or 5V. In this circuit, however,
Q3 and Q4 force the LTC1142 3.3V regulator into continuous
mode operation whenever 3.3V, 5V or 12V is programmed
at the VPP
OUT
pins of the LT1313. (See the LT1312 and
LT1313 data sheets for further detail.)
Burst Mode is a trademark of Linear Technology Corporation.
+
C1
68µF
D2
MBRS1100
15V AUX
SUPPLY
D3
MBRS130T3 T1*
15µH
R1
100Ω
R4
0.033Ω
3.37:1
D4
18V
+
C5
68µF
R2
100Ω
+
3.3V
OUTPUT
C3
220µF
Q4
2N7002
*CTX02-12753
COILTRONICS (407) 241-7876
1470/71 F04
9
LINER [ LINEAR TECHNOLOGY ]
