FAN5308_07 FAIRCHILD [Fairchild Semiconductor], FAN5308_07 Datasheet - Page 8

• Manufacturer Part Number: FAN5308_07
• Description: 800mA High-Efficiency Step-Down DC-DC Converter
• Manufacturer: FAIRCHILD [Fairchild Semiconductor]

©2005 Fairchild Semiconductor Corporation
FAN5308 Rev. 1.0.2
The N-channel transistor is turned off before the inductor
current becomes negative. At this time, the P-channel is
switched on again, starting the next pulse. The converter
continues these pulses until the high threshold (typical
1.6% above nominal value) is reached. A higher output
voltage in PFM mode gives additional headroom for the
voltage drop during a load transient from light to full load.
The voltage overshoot during this load transient is also
minimized due to active regulation during turn on of the
N-channel rectifier switch. The device stays in sleep
mode until the output voltage falls below the low thresh-
old. The FAN5308 enters the PWM mode as soon as the
output voltage can no longer be regulated in PFM with
constant peak current.
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:
where:
R
I
R
V
LOAD
dsON
L
OUT
= Inductor DC resistance
= Output current
= V
= P-channel switch on resistance
IN
– I
LOAD
(R
dsON
+ R
L
)
EQ. 2
8
UVLO and Soft Start
The reference and the circuit remain reset until the V
crosses its UVLO threshold.
The FAN5308 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 out-
put voltage overshoot. The soft-start is implemented as a
digital circuit, increasing the switch current in four steps
to the P-channel current limit (1500mA). Typical start-up
time for a 20µF output capacitor and a load current of
800mA is 800µs.
Short-Circuit Protection
The switch peak current is limited cycle-by-cycle to a typ-
ical value of 1500mA. 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 200mA.
Thermal Shutdown
When the die temperature exceeds 150°C, a reset
occurs and remains in effect until the die cools to 130°C.
At that time, the circuit is allowed to restart.
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IN