IRS2548DSPBF International Rectifier, IRS2548DSPBF Datasheet - Page 15

• Manufacturer Part Number: IRS2548DSPBF
• Description: IC, LED DRVR, SOIC
• Manufacturer: International Rectifier

Specifications of IRS2548DSPBF

• Topology: Boost (Step Up)
• No. Of Outputs: 1
• Output Current: 500mA
• Output Voltage: 16.6V
• Dimming Control Type: PWM
• Operating Temperature Range: -55°C To +150°C
• Driver Case Style: SOIC
• Switching Frequency: 44.5kHz
• Rohs Compliant: Yes
• Package: 14-lead SOIC
• Circuit: PFC Ballast Control and Half-Bridge Driver
• Offset Voltage (v): 600
• Output Source Current Min (ma): 500
• Output Sink Current Min (ma): 500
• Pbf: Yes
• V Bsuv+ (v): 9.0
• V Ccuv+ (v): 12.5
• V Ccuv- (v): 10.5
• T R (ns): 120
• T F (ns): 50
• Application: SMPS/Lighting

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II. PFC Section
Functional Description
In most LED drivers rated at more than a few Watts
high power factor high power factor (PC) is a
requirement. The driver needs to appear as a
resistive load to the AC input line voltage. The
degree to which the circuit matches a purely
resistive load is measured by the phase shift
between the input voltage and input current
harmonic distortion of the input current waveform.
The cosine of the phase angle between the input
voltage and input current is defined as the
displacement power factor and the amount of
harmonic distortion determines the distortion power
factor and total harmonic distortion (THD). The
overall power factor is the ratio between real and
apparent power and includes both displacement
and distortion. A power factor of 1.0 corresponds to
zero phase shift and a THD of 0% representing a
pure sinusoidal current waveform. In order to
provide a high PF and a low THD the IRS2548D
includes an active power factor correction (PFC)
circuit.
The control method implemented in the IRS2548D
is designed for a PFC Boost converter (Figure 8)
running in critical-conduction mode, the boundary
between continuous and discontinuous mode.
During the off period of each switching cycle of the
PFC MOSFET the circuit waits until the inductor
current falls to zero before turning the PFC
MOSFET on again. The PFC MOSFET is turned on
and off at a much higher frequency (>10KHz) than
the line input frequency (50 to 60Hz).
Figure 8: Boost converter circuit.
When the switch MPFC is turned on the inductor
LPFC is connected between the rectified line input
(+) and (-) causing the current in LPFC to rise
linearly. When MPFC is turned off LPFC is
connected between the rectified line input (+) and
the DC bus capacitor CBUS through diode DPFC
and the stored energy in LPFC supplies a current
into CBUS. MPFC is turned on and off at a high
frequency and the voltage on CBUS charges up to a
specified voltage. The feedback loop of the
IRS2548D regulates this voltage to a fixed value by
(+)
(-)
LPFC
MPFC
DPFC
+
CBUS
DC Bus
15
continuously monitoring the DC bus voltage and
adjusting the on-time of MPFC accordingly. For an
increasing DC bus the on-time is decreased and for a
decreasing DC bus the on-time is increased. This
negative feedback control is performed with a slow
loop speed such that the average inductor current
smoothly follows the low-frequency line input voltage
for high power factor and low THD. The on-time of
MPFC therefore appears to be fixed (except for on
time modulation which is discussed later) over several
cycles of the line voltage. With a fixed on-time and an
off-time
discharging to zero the switching frequency and duty
cycle vary to produce a high frequency near the zero
crossing of the AC input line voltage and a lower
frequency at the peak (Figure 9).
Figure 9: Sinusoidal line input voltage (solid
line), triangular PFC Inductor current and
smoothed sinusoidal line input current (dashed
line) over one half-cycle of the AC line input
voltage.
When the line input voltage is low (near the zero
crossing), the inductor current will charge to a
lower peak level and therefore the discharge time
will be fast resulting in a high switching frequency.
When the input line voltage is high (near the peak),
the inductor current will charge up to a higher
amount and the discharge time will be longer
giving a lower switching frequency.
The PFC control circuit of the IRS2548D (Figure
10) includes five control pins: VBUS, COMP, ZX,
PFC and OC. The VBUS pin measures the DC bus
voltage via an external resistor voltage divider. The
COMP pin voltage at the transconductance error
amplifier output sets the on-time of MPFC where
the speed of the feedback loop is determined by
the external COMP capacitor. The ZX input detects
when the inductor current has discharged to zero
each switching cycle using a secondary winding
from the PFC inductor. The PFC output provides
the gate driver output for the external MOSFET,
MPFC. The OC pin senses the current flowing
through MPFC and performs cycle-by-cycle over-
current protection.
V, I
determined
by
the
© 2011 International Rectifier
inductor
IRS2548D
t
current