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FC9911 Arkusz danych(PDF) 10 Page - First Silicon Co., Ltd

Numer części FC9911
Szczegółowy opis  LED Driver Switch-Mode LED Driver IC with High Current Accuracy
Download  13 Pages
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Producent  FS [First Silicon Co., Ltd]
Strona internetowa  http://www.firstsilicon.co.kr/
Logo FS - First Silicon Co., Ltd

FC9911 Arkusz danych(HTML) 10 Page - First Silicon Co., Ltd

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Reference
FC9911 includes a 2% accurate, 1.25V reference, which can
be used as the reference for the output current as well as
to set the switch current limit. This reference is also used
internally to set the over voltage protection threshold. The
reference is buffered so that it can deliver a maximum of
500µA external current to drive the external circuitry. The
reference should be bypassed with at least a 10nF low ESR
capacitor.
Note:
In order to avoid abnormal startup conditions, the bypass
capacitor at the REF pin should not exceed 0.22µF.
Oscillator
The oscillator can be set in two ways. Connecting the
oscillator resistor between the RT and GATE pins will
program the off-time. Connecting the resistor between RT
and GND will program the time period.
In both cases, resistor R
T sets the current, which charges
an internal oscillator capacitor. The capacitor voltage ramps
up linearly and when the voltage increases beyond the
internal set voltage, a comparator triggers the SET input of
.
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The time period of the oscillator can be computed as:
T
S ≈ RT • 11pF
Slope Compensation
For converters operating in the constant frequency mode,
slope compensation becomes necessary to ensure stability
of the peak current mode controller, if the operating duty
cycle is greater than 0.5. Choosing a slope compensation
which is one half of the down slope of the inductor current
ensures that the converter will be stable for all duty cycles.
Slope compensation can be programmed by two resistors
R
SLOPE and RSC. Assuming a down slope of DS (A/µs) for the
inductor current, the slope compensation resistors can be
computed as:
R
SLOPE = (10 • RSC) / (DS • 10
6
• T
S • RCS)
A typical value for R
SC is 499Ω.
Note:
The maximum current that can be sourced out of the SC
pin is 100µA. This limits the minimum value of the R
SLOPE
resistor to 25kΩ. If the equation for slope compensation
produces a value of R
SLOPE less than this value, then RSC
would have to be increased accordingly. It is recommended
that R
SLOPE be chosen in the range of 25 - 50kΩ.
Current Sense
The current sense input of the HV9911 includes a built in
100ns (minimum) blanking time to prevent spurious turn off
due to the initial current spike when the FET turns on.
The FC9911 includes two high-speed comparators - one is
used during normal operation and the other is used to limit
the maximum input current during input under voltage or
overload conditions.
The IC includes an internal resistor divider network, which
steps down the voltage at the COMP pin by a factor of
15. This stepped-down voltage is given to one of the
comparators as the current reference. The reference to the
other comparator, which acts to limit the maximum inductor
current, is given externally.
It is recommended that the sense resistor R
CS be chosen so
as to provide about 250mV current sense signal.
Current Limit
Current limit has to be set by a resistor divider from the
1.25V reference available on the IC. Assuming a maximum
operating inductor current I
pk (including the ripple current),
the voltage at the CLIM pin can be set as:
V
CLIM ≥ 1.2 • IPK • RCS + ( 5 • RSC / RSLOPE) • 0.9
Note that this equation assumes a current limit at 120%
of the maximum input current. Also, if V
CLIM is greater than
450mV, the saturation of the internal opamp will determine
the limit on the input current rather than the CLIM pin. In
such a case, the sense resistor R
CS should be reduced till
V
CLIM reduces below 450mV.
It is recommended that no capacitor be connected between
CLIM and GND.
Fault Protection
The FC9911 has built-in output over-voltage protection
and output short circuit protection. Both protection features
are latched, which means that the power to the IC must
be recycled to reset the IC. The IC also includes a FAULT
pin which goes low during any fault condition. At startup,
a monoshot circuit, (triggered by the POR circuit), resets
an internal flip-flop which causes FAULT to go high, and
remains high during normal operation. This also allows the
GATE drive to function normally. This pin can be used to
drive an external disconnect switch (Q2 in the Typical Boost
Application Circuit on pg.1), which will disconnect the load
during a fault condition. This disconnect switch is very
important in a boost converter, as turning off the switching
FC9911
2012. 10. 31
10/13
Revision No : 0


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