ISL1902FAZ-T7A Intersil, ISL1902FAZ-T7A Datasheet - Page 14

• Manufacturer Part Number: ISL1902FAZ-T7A
• Description: LED Lighting Drivers Dimmable AC Mains LED Driver PFC
• Manufacturer: Intersil

Specifications of ISL1902FAZ-T7A

• Rohs: yes
• Input Voltage: 4 V
• Operating Frequency: 320 Hz
• Maximum Supply Current: 14.5 mA
• Output Current: 1 A
• Maximum Operating Temperature: + 125 C
• Mounting Style: SMD/SMT
• Package / Case: QSOP-24
• Minimum Operating Temperature: - 40 C

Functional Description
Features
The ISL1902 LED driver is an excellent choice for low cost, AC
mains powered single conversion LED lighting applications. It
provides active power factor correction (PFC) to achieve high
power factor using critical conduction mode operation, and
incorporates additional features for compatibility with
triac-based dimmers. The ISL1902 includes support for both
PWM and DC current dimming of the output. Similar to the
ISL1901, the ISL1902 adds additional features to facilitate the
design of higher performance LED drivers.
Oscillator
The ISL1902 uses a critical conduction mode (CrCM) algorithm to
control the switching behavior of the converter. The ON-time of
the primary power switch is held virtually constant by the low
bandwidth control loop. The OFF-time duration is determined by
the time it takes the current or voltage to decay during the
flyback period. When the MMF (Magneto Motive Force) of the
magnetic element decays to zero (dB/dt=0), the winding
voltages collapse and the winding currents are zero (flyback) or
DC (SEPIC). Either may be monitored and used to initiate the next
switching cycle to achieve CrCM operation. Additionally, there is a
user adjustable threshold, DELADJ, to delay the initiation of the
next switching cycle to allow the drain-source voltage of the
primary switch to ring to a minimum. This allows quasi-ZVS
operation to reduce capacitive switching losses and improve
efficiency.
By its nature, the converter operation is variable frequency. There
are both minimum and maximum frequency clamps that limit
the range of operation. The minimum frequency clamp prevents
the converter from operating in the audible frequency range
while the maximum frequency clamp prevents operating at very
high frequencies that may result in excessive losses.
An individual switching period is the sum of the ON-time, the
OFF-time, and the restart delay duration. The ON-time is
determined by the control loop error voltage, VERR, and the
RAMP signal. As its name implies, the RAMP signal is a linearly
increasing signal that starts at zero volts and ramps to a
maximum of ~VERR/5 - 230mV. RAMP requires an external
resistor and capacitor connected to VREF to form an RC charging
network. If VERR is at its maximum level of VREF, the time
required to charge RAMP to ~850mV determines the maximum
ON-time of the converter. RAMP is discharged every switching
cycle when the ON-time terminates.
The OFF-time duration is determined by the design of the
magnetic element(s), which depends on the required energy
storage/transfer and the inductance of the windings. The
transformer/inductor design also determines the maximum
ON-time that can be supported without saturation, so, in reality,
the magnetic design is critical to every aspect of determining the
switching frequency range.
THE FLYBACK TOPOLOGY
The design methodology is similar to designing a discontinuous
mode (DCM) flyback transformer with the constraint that it must
operate at the DCM/CCM boundary at maximum load and
14
ISL1902
minimum input voltage. The difference is that the converter will
always operate at the DCM/CCM boundary, whereas a DCM
converter will be more discontinuous as the input voltage
increases or the load decreases. For PFC applications, the design
is further complicated by the input voltage waveform; a virtually
unfiltered rectified AC mains sinewave.
Once the output power, P
voltage, V
transformer design can be started. From the minimum AC input
voltage, the minimum average input voltage must be
determined. The converter behaves as if the input voltage is an
equivalent DC value due to the low control loop bandwidth. P
determines the amount of energy that must be stored in the
transformer on each switching cycle, but must be corrected for
efficiency. This includes leakage inductance losses, winding
losses, and all secondary side losses. This can be estimated as a
portion of the total efficiency,
all of the losses.
A typical minimum operating frequency and maximum duty cycle
must be selected. These are somewhat arbitrary in their
selection, but do ultimately determine core size. The typical
frequency is what occurs when the instantaneous rectified input
AC voltage is exactly at the equivalent DC value. The frequency
will be higher when the instantaneous input voltage is lower, and
lower when the instantaneous input voltage is higher. However,
the duty cycle at the equivalent DC input voltage determines the
ON-time for the entire AC half-cycle. The ON-time is constant due
to the low bandwidth control loop, but the OFF-time and duty
cycle vary with the instantaneous input voltage since the peak
switch current follows V = Ldi/dt.
The lowest frequency may require adjustment once the initial
calculations are complete to see if the operating frequency at the
peak of the minimum AC input voltage is acceptable.
P
V
IN
maxINrms
t
V
Ip(peak) = Peak primary current within a switching cycle
ON(MAX)
f
mINrms
typ(avg)
t
=
DELAY
D
Dmin = Minimum typical duty cycle
max
Nsp = Transformer turns ratio, Ns/Np
P
------ -
η
Lp = Primary inductance
Ls = Secondary inductance
t
o
t
OFF
η = Efficiency
ON
O
= Minimum RMS input voltage
= Maximum RMS input voltage
= Typical frequency when V
= Maximum typical duty cycle desired
= f
= User adjustable delay before the next switching cycle
, and the minimum input AC voltage are known, the
ON-time of the power FET controlled by OUT
OFF-time duration required for CrCM operation
begins
typ(avg)
W
TABLE 1. OSCILLATOR DEFINITIONS
x D
max
O
, the output current, I
η
, or as is typically done, includes
IN
(instantaneous) = V
O
, the output
March 20, 2013
IN
FN7981.2
(rms)
(EQ. 1)
O