IR3651SPBF
06/08/2011
Circuit Description
9
THEORY OF OPERATION
Introduction
The IR3651 is a voltage mode PWM
synchronous controller. The output voltage is set
by feedback pin (Fb) and the internal reference
voltage (1.25V). These are two inputs to error
amplifier. The error signal between these two
inputs 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 timing of the IC is controlled by an internal
oscillator circuit that can be externally
programmed up to 400kHz.
The IR3651 operates with a wide input voltage
from 12V to 100V 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
Vcc supply and assures that the IC doesn’t starts
until the Vcc reaches the set threshold. Lockout
occurs if Vcc falls below 4.1V. Normal operation
resumes once Vcc rises above the set value.
Shutdown
The output can be shutdown by pulling the soft-
start pin below 0.3V. This can be easily 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.
Error Amplifier
The IR3651 is a voltage mode controller. The
error amplifier is of transconductance type. The
amplifier is capable of operating with Type III
compensation control scheme using low ESR
output capacitance.
Operating Frequency Selection
The switching frequency is determined by
connecting an external resistor (Rt) to ground.
Figure 3 provides a graph of oscillator frequency
versus Rt.
Frequency Synchronization
The IR3651 is capable of accepting an external
digital synchronization signal. Synchronization
will be enabled by the rising edge at an external
clock. Switching frequency is set by external
resistor (Rt). During synchronization, Rt is
selected such that the free running frequency is
20% below the synchronization frequency.
When unused, the sync pin will remain floating
and is noise immune.
Pre-Bias Startup
IR3651 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. Below figure shows a typical Pre-
Bias condition at start up.
Depends on system configuration, specific
amount of output capacitors may be required to
prevent discharging the output voltage
Fig. 3: Switching Frequency vs. Rt
Switching Frequency vs. Rt
0
50
100
150
200
250
300
350
400
450
0 50 100 150 200 250 300
Rt (Kohm)
Switching Frequency (kHz)
Vo
Time
Volt
Pre-Bias Voltage
(Output Voltage before startup)
