LTC3723EGN-1 Linear Technology, LTC3723EGN-1 Datasheet - Page 15

• Manufacturer Part Number: LTC3723EGN-1
• Description: IC CTRLR PWM SYNC PUSHPLL 16SSOP
• Manufacturer: Linear Technology

Specifications of LTC3723EGN-1

• Pwm Type: Current Mode
• Number Of Outputs: 1
• Frequency - Max: 1MHz
• Duty Cycle: 50%
• Buck: No
• Boost: No
• Flyback: Yes
• Inverting: No
• Doubler: No
• Divider: No
• Cuk: No
• Isolated: Yes
• Operating Temperature: -40°C ~ 85°C
• Package / Case: 16-SSOP
• Frequency-max: 1MHz
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Voltage - Supply: -

OPERATIO
High Current Drivers
The LTC3723-1/LTC3723-2 high current, high speed driv-
ers provide direct drive of external power N-channel
MOSFET switches. The drivers swing from rail to rail. Due
to the high pulsed current nature of these drivers (1.5A
sink, 1A source), care must be taken with the board layout
to obtain advertised performance. Bypass V
minimum, low ESR, ESL ceramic capacitor. Connect this
capacitor with minimal length PCB leads to both V
GND. A ground plane is highly recommended. The driver
output pins (DRVA, DRVB) connect to the gates of the
external MOSFET switches. The PCB traces making these
connections should also be as short as possible to mini-
mize overshoot and undershoot of the drive signal.
Synchronous Rectification
The LTC3723-1/LTC3723-2 produces the precise timing
signals necessary to control secondary side synchronous
rectifier MOSFETs on SDRA and SDRB. Synchronous
rectifiers are used in place of Schottky or silicon diodes on
the secondary side to improve converter efficiency. As
MOSFET R
ciency improvements can be realized with synchronous
rectification, provided that the MOSFET switch timing is
optimized. Synchronous rectification also provides bipo-
lar output current capability, that is, the ability to sink as
well as source current.
Programming the Synchronous Rectifier
Turn-Off Delay
The LTC3723-1/LTC3723-2 controllers include a feature
to program the turn-off edge of the secondary side syn-
chronous rectifier MOSFETs relative to the beginning of a
DS(ON)
U
levels continue to drop, significant effi-
CC
with a 1µF
CC
and
new primary side power delivery pulse. This feature pro-
vides optimized timing for the synchronous MOSFETs
which improves efficiency. At higher load currents it
becomes more advantageous to delay the turn-off of the
synchronous rectifiers until the beginning of the new
power pulse. This allows for secondary freewheeling
current to flow through the synchronous MOSFET channel
instead of its body diode.
The turn-off delay is programmed with a resistor from
SPRG to GND, (Figure 13). The nominal regulated voltage
on SPRG is 2V. The external resistor programs a current
which flows out of SPRG. The delay can be adjusted from
approximately 20ns to 200ns, with resistor values of 10k
to 200k. Do not leave SPRG floating. The amount of delay
can also be modulated based on an external current
source that sinks current out of SPRG. Care must be taken
to limit the current out of SPRG to 350µA or less.
R
SPRG
SPRG
Figure 13. Synchronous Delay Circuitry
LTC3723-1/LTC3723-2
V
+
–
2V
+
–
TURN-OFF
SYNC OUT
372312 F13
15
372312f