SM8205 Datasheet, PDF (10/13 Pages) M.S. Kennedy Corporation – High-Power LED Driver with Integrated PWM Dimming MOSFET Driver

SM8205
Slope Compensation (SC)
The SM8205 uses an internal ramp voltage generator
(typically at 1.5VPEAK) for slope compensation, by an
external resistor connects SC pin to GND to generate a
respective ramp current source for slope compensation
use. In a fixed operating frequency, slope compensation
is necessary, Particularly, the operating duty cycle
greater than 0.5. choosing a slope compensation which
is one-half of the down slope of the inductor current to
ensure the stability operation.
Blanking
CS
100ns
1:2
Ramp
1.5V/T
Current Limit (CLIM)
The SM8205 features a current limit protection feature to
prevent any current runaway conditions. Current limit
has to be set by a resistor divider from the VCC (5V)
reference available on the IC. Assuming a maximum
operating inductor current IPK (including the ripple
current), the voltage at the CLIM pin can set as:
5 • RSC
( VCLIM ≥ 1.2 • IPK • RCS +
) • DMAX
RSLOPE
Note that this equation assumes a current limit at 120%
of the maximum input current. Also, if VCLIM is greater
than 450mV, the saturation of the internal E.A amplifier
will determine the limit on the input current rather than
the CLIM pin. In such a case, the sense resistor RCS
should be reduced till VCLIM reduces below 450mV.
SC
RSLOPE
Figure 3. The Functional Block of the Ramp Current
Slope compensation can be programmed by two
resistors RSLOPE and RSC. Assuming a down slope of
DS (A/uA) for the inductor current, the slope
compensation resistors can be computed as:
RSLOPE =
10 • RSC
6
DS • 10 • Ts • RCS
■ A typical value for RSC is 620Ω.
Current Sense (CS)
The current sense input of the SM8205 built-in a 100ns
(typical, see figure 3.) blanking time to prevent spurious
turn off due to the initial current spike when the external
power MOSFET turns on. SM8205 includes two high-
speed comparator – one is used during normal
operation, namely is PWM comparator, another one is
used to limit the maximum input current during input
under-voltage or overload conditions, which namely is
current-limit comparator.
The SM8205 integrates an internal resistor divider
network, which steps down the voltage at the COMP pin
by a factor of 7 (see IC’s block diagram). This steps
down voltage is fed to one of the PWM comparator as
the current reference. The reference to the other
comparators (current-limit comparator), which acts to
limit the maximum inductor current, is given externally.
It is recommended that the sense resistor RCS be
chosen so as to provide about 250mV current sense
signal.
www.samhop.com.tw
Internal Transconductance Error Amplifier
The SM8205 has a built-in transconductance amplifier
used to amplify the error signal inside the feedback loop.
The amplified current-sense signal is connected to the
negative input of the gm amplifier with the current
reference connected to IREF. The output of the op-amp
is controlled by the input at PWMD. When the signal at
PWMD is high, the output of the op-amp connects to
COMP; when the signal at PWMD is low, the output of
the op-amp disconnects from COMP to keep the charge
on the compensation capacitor. When the voltage at
PWMD goes high, the voltage on the compensation
capacitor forces the converter into a steady state. COMP
is connected to the negative input of the PWM
comparator with CMOS input through a buffer, forward
diode and 7:1 resistor divider, which draw a small
current from the compensation capacitor at COMP and
thus prevent discharge of the compensation capacitor
when the PWMD input is low.
FAULT Protection
The SM8205 includes a output over-voltage and LED
short-circuit protections. Both protection features are
latched, which means that the power to the IC must be
recycled to reset the IC. The IC also built-in a FAULT pin
which goes low during any fault conditions. At start-up, a
monoshot circuit (trigger by POR the circuit), reset an
internal SR filp-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 switch Q2 (see typical
application on page 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 MSOFET (Q1) during an output short-circuit
condition will not remove the fault (Q1 is not in the path
of the fault current), the disconnect switch will help to
disconnect the shorted load from the input.
10
Rev. 2.0 @ 2011/10