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LTC3900 Datasheet(PDF) 9 Page - Linear Technology

Part # LTC3900
Description  Synchronous Rectifier Driver for Forward Converters
Download  12 Pages
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Manufacturer  LINER [Linear Technology]
Direct Link  http://www.linear.com
Logo LINER - Linear Technology

LTC3900 Datasheet(HTML) 9 Page - Linear Technology

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LTC3900
9
3900i
APPLICATIO S I FOR ATIO
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+ and CSpins. If the inductor current
reverses into the Q4 causing CS+ to rise above CSby
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
RDS(ON). The current sense comparator is only active
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+ to rise above CS. The current
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 RDS(ON)
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,
RCS1 and RCS2 at the CS+ pin to increase the 10.5mV
threshold so that the LTC3900 operates in continuous
mode at no load.
The LTC3900 CS+ pin has an internal current sinking
clamp circuit (ZCS in the Block Diagram) that clamps the
pin to 11V. The clamp circuit is to be used together with the
external series resistor, RCS1 to protect the CS+ pin from
high Q4 drain voltage in the power transfer cycle. During
the power transfer cycle, Q4 is off, the drain voltage of Q4
is determined by the primary input voltage and the trans-
former turns ratio. This voltage can be high and may
damage the LTC3900 if CS+ is connected directly to the
drain of Q4. The current sinking capability of the clamp
circuit is 5mA minimum.
The value of the resistors, RCS1, RCS2 and RCS3 should be
calculated using the following formulas to meet both the
threshold and clamp voltage requirements:
k = {48 • IRIPPLE • RDS(ON)} –1
RCS2 = {200 • VIN(MAX) • NS/NP –2200 • (1 + k)} /k
RCS1 = k • RCS2
RCS3 = {RCS1 • RCS2} / {RCS1 + RCS2}
If k = 0 or less than zero, RCS2 is not needed and RCS1
= RCS3 = {VIN(MAX) • (NS/NP) – 11V} / 5mA
where:
IRIPPLE = Inductor peak-to-peak ripple current
RDS(ON) = On-resistance of Q4 at IRIPPLE/2
VIN(MAX) = Primary side main supply maximum input
voltage
NS/NP = Power transformer T1, turn ratio
SG
SYNC
FG
CG
3900 F06a
INDUCTOR
CURRENT
0A
CURRENT SENSE
COMPARATOR TRIP
SYNC
FG
CG
3900 F06b
INDUCTOR
CURRENT
0A
ADJUSTED CURRENT
SENSE THRESHOLD
Figure 6a. Discontinuous Current Mode Operation at No Load
Figure 6b. Continuous Current Mode Operation
with Adjusted Current Sense Threshold


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