FAN8301 Fairchild Semiconductor, FAN8301 Datasheet - Page 8

• Manufacturer Part Number: FAN8301
• Manufacturer: Fairchild Semiconductor

© 2008 Fairchild Semiconductor Corporation
FAN8301 • Rev. 1.0.0
Functional Description
The FAN8301 is a monolithic, non-synchronous,
current-mode, step-down regulator with internal power
MOSFETs. It achieves 2A continuous output current
over a wide input supply range from 4.75V to 16V with
excellent load and line regulation. The output voltage
can be regulated as low as 0.6V. The FAN8301 uses
current-mode operation that provides fast transient
response and eases loop stabilization. The FAN8301
requires a minimum number of readily available
standard external components.
Current-Mode PWM Control Loop
FAN8301 uses current-mode PWM control scheme.
The peak inductor current is modulated in each
switching cycle by an internal op-amp output signal to
achieve the output voltage regulation. An internal slope
compensation circuit is included to avoid sub-harmonic
oscillation at duty cycle greater than 50%. Current-
mode control provides cycle-by-cycle current limit
protection and superior regulation control loop response
than the traditional voltage-mode control.
In normal operation, the high-side MOSFET is turned on
at the beginning of each switching cycle, which causes
the current in the inductor to build up. The current-
control loop senses the inductor current by sensing the
voltage across the high-side senseFET during on time.
The output of the current-sense amplifier is summed
with the slope compensation signal and the combined
signal is compared with the error amplifier output to
generate the PWM signal. As the inductor current
ramps up to the controlled value, the high-side
MOSFET is turned off and the inductor current reaches
zero through a freewheeling diode. In light-load
condition, the high-side switch may be kept off for
several cycles to improve efficiency.
Short-Circuit Protection
The FAN8301 protects output short circuit by switching
frequency fold-back.
FAN8301 is reduced to about 45kHz when the output is
shorted to ground. This frequency fold-back allows the
inductor current more time to decay to prevent potential
run-away condition. The oscillator frequency switches to
370kHz as V
regulated level.
Slope Compensation and Inductor Peak
Current
The slope compensation provides stability in constant
frequency architecture by preventing sub-harmonic
oscillations at high duty cycles. It is accomplished
internally by adding a compensating ramp to the
inductor current signal at duty cycles in excess of 50%.
OUT
rises gradually from 0V back to
The
oscillator
frequency of
8
Maximum Load Current at Low V
The FAN8301 is able to operate with input supply
voltage as low as 4.75V, although the maximum
allowable output current is reduced as a function of duty
cycle (see Figure 15). Additionally, at this low input
voltage; if the duty cycle is greater than 50%, slope
compensation reduces allowable output current.
Inductor Selection
A higher inductor value lowers ripple current. The
inductor value can be calculated as:
where:
f
V
V
ΔI
Considering worst case, the equation is changed to:
Input Capacitor Selection
To prevent high-frequency switching current passing to
the input, the input capacitor impedance at the
switching frequency must be less than input source
impedance. High-value, small, inexpensive, lower-ESR
ceramic capacitors are recommended. 10µF ceramic
capacitors should be adequate for 2A applications.
Output Capacitor Selection
A larger output capacitor value keeps the output ripple
voltage smaller. The formula of output ripple ΔV
where C
equivalent series resistance of the output capacitor.
Output Voltage Programming
The output voltage is set by a resistor divider, according
to the following equation:
V
s
L
L
Δ
OUT
OUT
IN
is the switching frequency;
L
V
=
=
Is the inductor ripple current.
is the input supply voltage; and
OUT
f
f
is the output voltage;
S
=
V
S
OUT
0
Δ ⋅
⋅
V
≅
6 .
Δ
OUT
OUT
I
I
Δ
⎛
⎜
⎝
L
L
1
I
,
L
MAX
+
⎛
⎜
⎜
⎝
⎛
⎜
⎜
⎝
1
is the output capacitor and ESR is the
R
R
ESR
−
2
3
V
⎛
⎜
⎜
⎝
⎞
⎟
⎠
V
1
OUT
+
IN
−
V
8
V
IN
⋅
⎞
⎟
⎟
⎠
C
OUT
,
OUT
MAX
1
⋅
⎞
⎟
⎟
⎠
f
S
⎞
⎟
⎟
⎠
IN
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OUT
is:
(1)
(2)
(3)
(4)