FAN2011EMPX_NL Fairchild Semiconductor, FAN2011EMPX_NL Datasheet - Page 7

• Manufacturer Part Number: FAN2011EMPX_NL
• Description: High-efficiency step-down DC/DC converter
• Manufacturer: Fairchild Semiconductor

FAN2011/FAN2012 Rev. 1.0.4
Block Diagram
Detailed Operation Description
The FAN2011 is a step-down pulse width modulated
(PWM) current mode converter with a typical switching
frequency of 1.3MHz. At the beginning of each clock
cycle, the P-channel transistor is turned on. The inductor
current ramps up and is monitored via an internal circuit.
The P-channel switch is turned off when the sensed cur-
rent causes the PWM comparator to trip when the output
voltage is in regulation or when the inductor current
reaches the current limit (set internally, typically
2600mA). 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 solely given by the ratio of output voltage
and input voltage. Therefore, the converter runs with a
minimum duty cycle when output voltage is at minimum
and input voltage is at maximum.
100% Duty Cycle Operation
As the input voltage approaches the output voltage and
the duty cycle exceeds the typical 95%, the converter
turns the P-channel transistor continuously on. In this
mode, the output voltage is equal to the input voltage
minus the voltage drop across the P-channel transistor:
V
R
I
R
LOAD
OUT
DS_ON
L
= Inductor DC resistance
= V
= Output current
= P-channel switch ON resistance
IN
– I
LOAD
FB
GND
0.8V
(R
SOFT START
DS_ON
DIGITAL
SLOPE COMPENSATION
IS
ERROR
AMP
+ R
OSC
L
), where
Figure 4. Block Diagram
COMP
VOLTAGE
REF
COMP
OVER
FB
7
UVLO and Soft Start
The reference and the circuit remain reset until the V
crosses its UVLO threshold.
The FAN2011 has an internal soft-start circuit that limits
the in-rush current during start-up. This prevents possi-
ble 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 (2600mA). Typical
start-up time for a 40µF output capacitor and a load cur-
rent of 1500mA is 800µs.
Short Circuit Protection
The switch peak current is limited cycle by cycle to a typ-
ical value of 2600mA. In the event of an output voltage
short circuit, the device operates with a frequency of
400kHz and minimum-duty cycle, therefore the average
input current is typically 350mA.
Thermal Shutdown
When the die temperature exceeds 150°C, a reset
occurs and remains in effect until the die cools to 130°C,
at which point, the circuit restarts.
CONTROL
EN
LOGIC
UNDER VOLTAGE
LOCKOUT
MOSFET
DRIVER
IS
CURRENT
SENSE
GND
V
IN
SW
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IN