LTC1922EG-1 Linear Technology, LTC1922EG-1 Datasheet - Page 17

• Manufacturer Part Number: LTC1922EG-1
• Description: IC CTRLR PWM SYNC 20SSOP
• Manufacturer: Linear Technology

Specifications of LTC1922EG-1

• Pwm Type: Voltage/Current Mode
• Number Of Outputs: 1
• Frequency - Max: 1MHz
• Duty Cycle: 99%
• Buck: No
• Boost: No
• Flyback: No
• Inverting: No
• Doubler: No
• Divider: No
• Cuk: No
• Isolated: Yes
• Operating Temperature: -40°C ~ 85°C
• Package / Case: 20-SSOP
• Frequency-max: 1MHz
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Voltage - Supply: -

OPERATIO
Phase Modulator
The LTC1922-1 phase modulation control circuitry is
comprised of the phase modulation comparator and logic,
the error amplifier, and the soft-start amplifier (see
Figure 11). Together, these elements develop the required
phase overlap (duty cycle) required to keep the output
voltage in regulation. In isolated applications, the sensed
output voltage error signal is fed back to COMP across the
input to output isolation boundary by an optical coupler
and shunt reference/error amplifier (LT
tion. The FB pin is connected to GND, forcing COMP high.
The collector of the optoisolator is connected to COMP
directly. The voltage COMP is internally attenuated by the
LTC1922-1. The attenuated COMP voltage provides one
input to the phase modulation comparator. This is the
current command. The other input to the phase modula-
tion comparator is the RAMP voltage, level shifted by
approximately 400mV. This is the current loop feedback.
During every switching cycle, alternate diagonal switches
U
COMP
RAMP
R
LEB
SS
FB
12 A
1.2V
V
REF
+
–
+
–
ERROR
AMPLIFIER
SOFT-START
AMPLIFIER
BLANKING
®
1431) combina-
Figure 11. Phase Modulation Circuitry
50k
14.9k
–
+
COMPARATOR
MODULATION
PHASE
400mV
Q
–
+
S
R
(MA-MD or MB-MC) conduct and cause current in an
output inductor to increase. This current is seen on the
primary of the power transformer divided by the turns
ratio. Since the current sense resistor is connected
between GND and the two bottom bridge transistors, a
voltage proportional to the output inductor current will be
seen across R
connected to RAMP and CS, usually through a small
resistor (R
either COMP/5.2 – 400mV, or 400mv, the overlap conduc-
tion period will terminate. During normal operation, the
attenuated COMP voltage will determine the RAMP/CS trip
point. During start-up, or slewing conditions following a
large load step, the 400mV CS threshold will terminate the
cycle, as COMP will be driven high, such that the attenu-
ated version exceeds the 400mV threshold. In extreme
conditions, the 600mV threshold on CS will be exceeded,
invoking a soft-start/restart cycle.
CLK
FROM
CURRENT
LIMIT
COMPARATOR
CLK
SLOPE
CLK
SENSE
MODULATION
). When the voltage on RAMP/CS exceeds
TOGGLE
PHASE
LOGIC
S Q
R
F/F
. The high side of R
Q
Q
1922 F11
LTC1922-1
A
B
C
D
SENSE
17
is also