ir3802 International Rectifier Corp., ir3802 Datasheet - Page 8

• Manufacturer Part Number: ir3802
• Description: Wide-input Voltage, Synchronous Buck Regulator
• Manufacturer: International Rectifier Corp.

11/04/08
Circuit Description
THEORY OF OPERATION
The
synchronous regulator and operates with a fixed
600kHz switching frequency, allowing the use of
small external components.
The output voltage is set by feedback pin (Fb)
and the internal reference voltage (0.6V). These
are two inputs to error amplifier. The error signal
between these two inputs is amplified and it is
compared to a fixed frequency linear sawtooth
ramp.
A trailing edge modulation is used for generating
fixed frequency pulses (PWM) which drives the
internal N-channel MOSFETs.
The internal oscillator circuit uses on-chip
circuitry, eliminating the need for external
components.
The IR3802 operates with single input voltage
from 4.5V to 14V allowing an extended operating
input voltage range.
The over-current protection is performed by
sensing current through the R
MOSFET. This method enhances the converter’s
efficiency and reduces cost by eliminating a
current sense resistor. The current limit is
programmable by using an external resistor.
Under-Voltage Lockout
The under-voltage lockout circuit monitors the
two input supplies (Vcc and Vc) and assures that
the MOSFET driver outputs remain in the off
state whenever the supply voltage drops below
set thresholds. Lockout occurs if Vcc or Vc fall
below 4.3V and 3.3V respectively. Normal
operation resumes once Vcc and Vc rise above
the set values.
Thermal Shutdown
Temperature sensing is provided inside the
IR3802. The trip threshold is typically set to
140
shutdown turns off both MOSFETs. Thermal
shutdown is not latched and automatic restart is
initiated when the sensed temperature drops
within the operating range. There is a 20
hysteresis in the thermal shutdown threshold.
o
C. When trip threshold is exceeded, thermal
IR3802
is
a
voltage
DS(on)
mode
of low side
PWM
o
C
Pre-Bias Startup
The IR3802 is able to start up into pre-charged
output,
disturbances of the output voltage.
The output starts in asynchronous fashion and
keeps the synchronous MOSFET off until the first
gate signal for control MOSFET is generated.
Figure 4 shows a typical Pre-Bias condition at
start up.
Depending on system configuration, a specific
amount of output capacitors may be required to
prevent discharging the output voltage.
Shutdown
The output can be shutdown by pulling the soft-
start pin below 0.3V. This can easily be done by
using an external small signal transistor. During
shutdown both MOSFET drivers will be turned
off. Normal operation will resume by cycling soft
start pin.
Pre-Bias Voltage
V
which
Fig. 4: Pre-Bias start up
prevents
IR3802MPBF
oscillation
PD-60356
Time
Vo
and
8