LTC1698 LINER [Linear Technology], LTC1698 Datasheet - Page 10

• Manufacturer Part Number: LTC1698
• Description: Isolated Secondary Synchronous Rectifier Controller
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
LTC1698
Undervoltage Lockout
In UVLO (low V
off and the pins OPTODRV, V
forced low. The LTC1698 allows the bandgap and the
internal bias currents to reach their steady-state values
before releasing UVLO. Typically, this happens when V
reaches approximately 4.0V. Beyond this threshold, the
drivers start switching. The OPTODRV, V
I
fully functional. However, if the V
the OPTODRV and V
limited. See the OPTO driver graphs in the Typical Perfor-
mance Characteristics section.
V
The bias supply for the LTC1698 is generated by peak
rectifying the isolated transformer secondary winding. As
shown in Figure 2, the zener diode Z1 is connected from
base of Q5 to ground such that the emitter of Q5 is
regulated to one diode drop below the zener voltage. R
selected to bring Z1 into conduction and also provide base
current to Q5. A resistor (on the order of a few hundred
ohms), in series with the base of Q5, may be required to
surpress high frequency oscillations depending on Q5’s
selection. A power MOSFET can also be used by increasing
the zener diode value to offset the drop of the gate-to-
source voltage. V
supply voltage, driver load and clock frequency. A 4.7 F
bypass capacitor for the V
applications. This capacitor must be large enough to
provide a stable DC voltage to meet the LTC1698 V
10
COMP
DD
Regulator
pins return to their normal values and the chip is
V
SECONDARY
DD
*R
DD
B
1
Figure 2. V
voltage) the drivers FG and CG are shut
D3
IS OPTIONAL, SEE TEXT
0.47 F
U
supply current varies linearly with the
AUX
current sourcing capabilities are
U
DD
R
2k
Z
Z1
10V
DD
supply is sufficient for most
AUX
R
Regulator
B
*
DD
, PWRGD and I
voltage is less than 7V,
W
Q5
FZT690
1698 F02
4.7 F
AUX
V
DD
, PWRGD and
U
COMP
are
Z
DD
DD
is
V
supply requirement. Under start-up conditions, it must be
small enough to power up instantaneously, enabling the
LTC1698 to regulate the feedback loop. Using a larger
capacitor requires evaluation of the start-up performance.
SYNC Input
Figure 3 shows the synchronous forward converter appli-
cation. The primary controller LT3781 runs at a fixed
frequency and controls MOSFETs Q1 and Q2. The second-
ary controller LTC1698 controls MOSFETs Q3 and Q4. An
inexpensive, small-size pulse transformer T2 synchro-
nizes the primary and the secondary controllers. Figure 4
shows the pulse transformer timing waveforms. When the
LT3781 synchronization output SG goes low, MOSFET
IN
CONTROLLER
SYNC
PRIMARY
PRIMARY
LT3781
Figure 3. Synchronization Using Pulse Transformer
TG
BG
SG
CG
FG
SG
Figure 4. Primary Side and Secondary Side
Synchronization Waveforms
TG
BG
C
Q2
D1
SG
Q1
ISOLATION BARRIER
D2
T2
•
•
•
•
T1
C
SYNC
Q4
Q3
R
SYNC
CG
FG SYNC
CONTROLLER
SECONDARY
SECONDARY
LTC1698
L1
1698 F03
1698 F04
V
1698f
C
OUT
OUT