LTC3722-1 LINER [Linear Technology], LTC3722-1 Datasheet - Page 18

• Manufacturer Part Number: LTC3722-1
• Description: Synchronous Dual Mode Phase Modulated Full Bridge Controllers
• Manufacturer: LINER [Linear Technology]

LTC3722-1/LTC3722-2
OPERATIO
of slope compensation. The value of this resistor is depen-
dent on several factors including minimum V
switching frequency, current sense resistor value and
output inductor value. An illustrative example with the
design equation is provided below.
Example: V
18
Transformer turns ratio (N) = V
V
R
f
R
(2 • 2.2 A • 100k • 82.5 A • 3)
R
to account for tolerances in I
SW
OUT
CS
SLOPE
SLOPE
= 300kHz, i.e., transformer f = f
= 0.025
= 3
C
T
V
I
L = 2.2 H
= V
= 505 , choose the next higher standard value
OUT
Figure 9. Slope Compensation Circuitry
IN
OUT
O
= 36V to 72V
= 40A
• R
= 3.3V
LTC3722
U
CS
33k
/(2 • L • f
I =
R
CS
V(C
CS
33k
T
)
T
270mV
650mV
• 82.5 A • N) = 3.3V • 0.025/
CS
SLOPE
ADDED
SLOPE
CURRENT SENSE
PULSE BY PULSE
CURRENT LIMIT
CURRENT LIMIT
WAVEFORM
IN(MIN)
R
OVERLOAD
SLOPE
+
–
+
–
, R
SW
Figure 10. Current Sense/Fault Circuitry Detail
CS
MOD
ENABLE
UVLO
• D
/2 = 150kHz
, N and L.
MAX
3722 F07
R
BRIDGE
CURRENT
CS
IN
/
, V
OUT
LATCH
PWM
S
S Q
R
Q
,
PWM
LOGIC
Current Sensing and Overcurrent Protection
Current sensing provides feedback for the current mode
control loop and protection from overload conditions. The
LTC3722-1/LTC3722-2 are compatible with either resis-
tive sensing or current transformer methods. Internally
connected to the LTC3722-1/LTC3722-2 CS pin are two
comparators that provide pulse-by-pulse and overcurrent
shutdown functions respectively. (See Figure 10)
The pulse-by-pulse comparator has a 270mV nominal
threshold. If the 270mV threshold is exceeded, the PWM
cycle is terminated. The overcurrent comparator is set
approximately 2x higher than the pulse-by-pulse level. If
the current signal exceeds this level, the PWM cycle is
terminated, the soft-start capacitor is quickly discharged
and a soft-start cycle is initiated. If the overcurrent condi-
tion persists, the LTC3722-1/LTC3722-2 halts PWM op-
eration and waits for the soft-start capacitor to charge up
to approximately 4V before a retry is allowed. The soft-
start capacitor is charged by an internal 12 A current
source. If the fault condition has not cleared when soft-
start reaches 4V, the soft-start pin is again discharged and
a new cycle is initiated. This is referred to as hiccup mode
operation. In normal operation and under most abnormal
conditions, the pulse-by-pulse comparator is fast enough
to prevent hiccup mode operation. In severe cases, how-
ever, with high input voltage, very low R
and a shorted output, or with saturating magnetics, the
overcurrent comparator provides a means of protecting
the power converter.
ENABLE
UVLO
–
+
+
–
S Q
R
Q
0.4V
4.1V
Q
H = SHUTDOWN
OUTPUTS
12 A
3722 F08
SS
C
SS
DS(ON)
MOSFETs
372212i