ir3637s International Rectifier Corp., ir3637s Datasheet - Page 4

• Manufacturer Part Number: ir3637s
• Description: 1% Accurate Synchronous Pwm Controller
• Manufacturer: International Rectifier Corp.

IR3637SPBF
BLOCK DIAGRAM
THEORY OF OPERATION
Introduction
The IR3637 is a fixed frequency, voltage mode synchro-
nous controller and consists of a precision reference
voltage, an error amplifier, an internal oscillator, a PWM
comparator, 0.5A peak gate driver, soft-start and shut-
down circuits (see Block Diagram).
The output voltage of the synchronous converter is set
and controlled by the output of the error amplifier; this is
the amplified error signal from the sensed output voltage
and the reference voltage.
This voltage is compared to a fixed frequency linear
sawtooth ramp and generates fixed frequency pulses of
variable duty-cycle, which drives the two N-channel ex-
ternal MOSFETs.The timing of the IC is provided through
an internal oscillator circuit which uses on-chip capaci-
tor to set the oscillation frequency to 400kHz.
4
SS/SD
Comp 7
POR
Fb 1
8
0.8V
3V
25uA
25K
25K
64uA Max
Figure 2 - Simplified block diagram of the IR3637.
Error Amp
3.3V
4.2V
Vcc
Vc
0.4V
Ct
www.irf.com
Oscillator
FbLo Comp
Error Comp
Short-Circuit Protection
The output is protected against the short-circuit. The
IR3637 protects the circuit for shorted output by sens-
ing the output voltage (through the external resistor di-
vider). The IR3637 shuts down the PWM signals, when
the output voltage drops below 0.4V.
Under-Voltage Lockout
The under-voltage lockout circuit assures that the
MOSFET driver outputs remain in the off state whenever
the supply voltage drops below set parameters. Lockout
occurs if Vc or Vcc fall below 3.3V and 4.2V respec-
tively. Normal operation resumes once Vc and Vcc rise
above the set values.
Shutdown
The converter can be shutdown by pulling the soft-start
pin below 0.4V. This can be easily done by using an
external small signal transistor. During shutdown the con-
trol MOSFET driver is turned off and the synchronous
MOSFET driver is turned on.
POR
Generator
Bias
Reset Dom
S
R
Q
3V
POR
0.8V
06/16/05
6
5
3
4
2
Rev.1.1
LDrv
Vcc
Vc
HDrv
Gnd