IRDC3629 International Rectifier, IRDC3629 Datasheet - Page 8

• Manufacturer Part Number: IRDC3629
• Description: BOARD EVAL SYNC BUCK CONTROLLER
• Manufacturer: International Rectifier

Circuit Description
THEORY OF OPEARTION
Introduction
The IR3629/29A is a voltage mode PWM
synchronous controller and operates with a fixed
600kHz
frequency, allowing the use of small external
components. The output voltage is set by a
feedback pin (Fb) and the internal reference
voltage (0.6V). These are the two inputs to the
error amplifier. The error signal between these
two inputs is amplified and it is compared to a
fixed frequency linear sawtooth ramp and
generates fixed frequency pulses of variable
duty-cycle (D) which drivers N-channel external
MOSFETs.
The internal oscillator circuit uses an on-chip
capacitor to set the switching frequency.
The IR3629/29A operates with single input
voltage from 4.5V to 12V allowing an extended
operating input voltage range.
The current limit is programmable and uses on-
resistance of the low-side MOSFET, eliminating
the need for an external current sense resistor
Under-Voltage Lockout
The under-voltage lockout circuit monitors the
two input supplies (Vcc and Vc) and ensures that
the MOSFET driver outputs remain in the off
state whenever the supply voltage drops below
set thresholds. Lockout occurs if Vc or Vcc fall
below 3.3V and 4.2V respectively. Normal
operation resumes once Vc and Vcc rise above
the set values.
Thermal Shutdown
Temperature sensing is provided inside the
IR3629/29A. The trip threshold is typically set to
145
thermal shutdown discharges the Soft Start
voltage and 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.
11/29/2007
o
C. When the trip threshold is exceeded,
(300kHz
for
IR3629A)
switching
.
o
C
(Output Voltage before startup)
Power Good
The IR3629/29A provides an open collector
power good signal which reports the status of the
output. The output is sensed through the
dedicated Vsns pin. The power good threshold
can be externally programmed using two external
resistors.
internally set to 0.38V (typical).
Minimum Pulse Width
The time required for turning on and off the high
side MOSFET is defined as “Minimum Pulse
Width”. To ensure that a reliable operation is
achieved the following condition needs to be met:
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 the
soft start pin.
Pre-Bias Startup
The IR3629/29A is able to start up into pre-
charged output, which prevents oscillation and
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
startup.
Depending on the system configuration, specific
amount of output capacitors may be required to
prevent discharging the output voltage.
Pre-Bias Voltage
IR3629/IR3629A MPbF
Volt
The power
T
on
(min)
Fig. 4: Pre-Bias startup
<
V
in
(max)
V
out
*
good comparator
F
s
Time
Vo
is
8