FAN8303 데이터 시트보기 (PDF) - Fairchild Semiconductor
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FAN8303 Datasheet PDF : 12 Pages
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Functional Description
The FAN8303 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 5V to 23V with
excellent load and line regulation. The output voltage
can be regulated as low as 0.6V. The FAN8303 uses
current-mode operation that provides fast transient
response and eases loop stabilization. The FAN8303
requires a minimum number of readily available
standard external components.
Current Mode PWM Control Loop
FAN8303 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
compared to the traditional voltage-mode control.
Inductor Selection
A higher inductor value lowers ripple current. The
inductor value can be calculated as:
L
=
VOUT
fS ⋅ ΔIL
⎜⎜⎝⎛1
−
VOUT
VIN
⎟⎟⎠⎞
(1)
where:
fs is the switching frequency;
VOUT is the output voltage;
VIN is the input supply voltage; and
ΔIL Is the inductor ripple current.
Considering worst case, the equation is changed to:
L=
fS
VOUT
⋅ ΔIL,MAX
⎜⎜⎝⎛1
−
VOUT
VIN,MAX
⎟⎞
⎟⎠
(2)
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.
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 ΔVOUT is:
Short-Circuit Protection
The FAN8303 protects output short circuit by switching
frequency fold-back. The oscillator frequency 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 VOUT rises gradually from 0V back to 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%.
Maximum Load Current at Low VIN
The FAN8303 is able to operate with input supply
voltage as low as 5V, 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.
ΔVOUT
≅ ΔIL ⎜⎜⎝⎛ESR +
1
8 ⋅COUT
⋅ fS
⎟⎟⎠⎞
(3)
where COUT is the output capacitor and ESR is the
equivalent series resistance of the output capacitor.
Output Voltage Programming
The output voltage is set by a resistor divider, according
to the following equation:
VOUT
= 0.6⎜⎛1+
⎝
R2 ⎟⎞
R3 ⎠
(4)
Freewheeling Diode
An output freewheeling diode carries load current when
the high-side switch is turned off. Therefore, use a
Schottky diode to reduce loss due to diode forward
voltage and recovery time. The diode should have at
least 2A current rating and a reverse blocking voltage
greater than the maximum input voltage. The diode
should be close to the SW node to keep traces short
and reduce ringing.
© 2008 Fairchild Semiconductor Corporation
FAN8303 • Rev. 1.0.0
8
www.fairchildsemi.com
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