ltc3900 Linear Technology Corporation, ltc3900 Datasheet - Page 8

• Manufacturer Part Number: ltc3900
• Description: Synchronous Rectifier Driver For Forward Converters
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
LTC3900
threshold of the comparators. This can cause only one of
the SYNC comparators to trip. This also appears as
incorrect SYNC pulse and the timer will not reset.
The timeout period is determined by the external R
C
achieved by making the timeout threshold a ratio of V
The ratio is 0.2x, set internally by R1 and R2 (see Figure 3).
The timeout period should be programmed to be around
1 period of the primary switching frequency using the
following formula:
To reduce error in the timeout setting due to the discharge
time, select C
a C
R
LTC3900 with minimum PCB trace between C
TIMER pin and GND. This is to reduce any ringing caused
by the PCB trace inductance when C
ringing may introduce error to the timeout setting.
The timer input also includes a current sinking clamp
circuit (Z
0.5 • V
clamp circuit prevents the timer cap from getting fully
charged up to the rail, which results in a longer discharge
time. The current sinking capability of the circuit is around
1mA. The timeout function can be disabled by connecting
the timer pin to GND.
8
TMR
TMR
TIMEOUT = 0.2 • R
TMR
Figure 5. Timer Waveforms with Incorrect SYNC Pulses
TIMER RESET
. C
values and is independent of the V
(INTERNAL)
CC
TMR
around 470pF and then calculate the required
TIMER
TMR
if there is missing SYNC/timer reset pulse. This
SYNC
THRESHOLD
FG
CG
MISSING/LOW
TIMEOUT
should be placed as close as possible to the
SYNC PULSE
TMR
in Figure 3) that clamps this pin to about
POSITIVE
between 100pF and 1000pF. Start with
TIMEOUT
U
TMR
TIMER DO NOT RESET
AT SECOND NEGATIVE
SYNC PULSE
U
• C
TMR
+ 0.27E-6
W
TMR
TIMER RESET AFTER
RECEIVING POSITIVE
SYNC PULSE
CC
discharges. This
voltage. This is
3900 F05
U
TMR
TMR
, the
and
CC
.
Current Sense
The differential input current sense comparator is used for
sensing the voltage across the drain-to-source terminals
of Q4 through the CS
reverses into the Q4 causing CS
more than 10.5mV, the LTC3900 pulls CG low. This
comparator is used to prevent inductor reverse current
buildup during power down or Burst Mode operation,
which may cause damage to the MOSFET. The 10.5mV
input threshold has a positive temperature coefficient,
which closely matches the TC of the external MOSFET
R
250ns after CG goes high; this is to avoid any ringing
immediately after Q4 is switched on.
Under light load conditions, if the inductor average current
is less than half of its peak-to-peak ripple current, the
inductor current will reverse into Q4 during a portion of the
switching cycle, forcing CS
sense comparator input threshold is set at 10.5mV to
prevent tripping under light load conditions. If the product
of the inductor negative peak current and MOSFET R
is higher than 10.5mV, the LTC3900 will operate in discon-
tinuous current mode. Figure 6 shows the LTC3900 oper-
ating in discontinuous current mode; the CG output goes
low before the next negative SYNC pulse, as soon as the
inductor current becomes negative. Discontinuous cur-
rent mode is sometimes undesirable. To disable discon-
tinuous current mode operation, add a resistor divider,
R
threshold so that the LTC3900 operates in continuous
mode at no load.
DS(ON)
CS1
Figure 6a. Discontinuous Current Mode Operation at No Load
INDUCTOR
CURRENT
and R
. The current sense comparator is only active
SYNC
SG
CG
FG
CS2
at the CS
+
and CS
COMPARATOR TRIP
+
CURRENT SENSE
+
to rise above CS
pin to increase the 10.5mV
–
pins. If the inductor current
+
to rise above CS
–
. The current
3900 F06a
0A
DS(ON)
–
3900f
by