FAN2013B Fairchild Semiconductor, FAN2013B Datasheet - Page 6

• Manufacturer Part Number: FAN2013B
• Description: The FAN2013B is a high-efficiency, low-noise, synchronous Pulse Width Modulated (PWM) current-mode DC-DC converter designed for low-voltage applications
• Manufacturer: Fairchild Semiconductor

© 2008 Fairchild Semiconductor Corporation
FAN2013B • Rev. 1.0.2
Block Diagram
Detailed Operation Description
The FAN2013B is a step-down pulse-width modulated
(PWM) current mode converter with a fixed switching
frequency of 1.2MHz. At the rising edge of each clock
cycle, the P-channel transistor is turned on until the
PWM comparator trips or the current limit is reached.
During the ON time, the inductor current ramps up and
is monitored by the internal current-mode control loop.
After a minimum dead time, the N-channel transistor is
turned ON and the inductor current ramps down. As the
clock cycle is completed, the N-channel switch is turned
OFF and the next clock cycle starts. The duty cycle is
given by the ratio of output voltage and input voltage.
The converter runs at minimum duty cycle when the
output voltage is at minimum and input voltage is at
maximum. The duty cycle increases proportionally as
the input voltage approaches the output voltage.
UVLO and Soft-Start
The internal voltage reference, V
reset until V
The FAN2013B has an internal soft-start circuit that
limits the inrush current during start-up. This prevents
possible voltage drops of the input voltage and
eliminates the output voltage overshoot. The soft-start
is implemented as a digital circuit, increasing the switch
current in four steps to the P-channel current limit.
Typical start-up time for a 40µF output capacitor with a
load current of 2.0A is 800µs.
IN
reaches the UVLO threshold.
REF
, and the IC remain
Figure 10.
6
Block Diagram
Output Over-Voltage Protection
When output voltage, V
above the nominal value, the device turns OFF the P-
channel switch and turns ON part of the N-channel
transistor with a built-in current limit of about 400mA.
When V
device starts switching normally in closed loop. If output
voltage is pulled up by an external voltage source with a
current limit higher than typical 400mA, the output
voltage stays up at the external voltage source level.
The over-voltage protection is designed to limit the
output voltage excursion in case of a transient response
from full load to a minimum load.
Output Short-Circuit Protection
The switch peak current is limited cycle by cycle to a
typical value of 3.5A. In the event of an output voltage
short circuit, the device operates with a frequency of
400kHz and minimum duty cycle, making the average
typical input current 0.45A.
Thermal Shutdown
When the die temperature exceeds 150°C, a reset
occurs and remains in effect until the die cools to
130°C, when the circuit is allowed to re-initiate a soft-
start cycle..
OUT
reaches the hysteresis of about 2%, the
OUT
, reaches approximately 7%
www.fairchildsemi.com