ISL1903 Datasheet, PDF (14/19 Pages) Intersil Corporation – Dimmable Buck LED Driver - AC Mains or DC Input LED Driver

ISL1903
V_mINrms =
V_maxINrms =
f_min(avg) =
D_max =
D_min =
t_ON(MAX) =
t_ON
t_OFF
L=
Nsp =
I_p(peak) =
t_delay =
TABLE 1. OSCILLATOR DEFINITIONS
Minimum RMS input voltage
Maximum RMS input voltage
Typical frequency when V_IN (instantaneous) =
minimum V_IN(rms)
Maximum typical duty cycle desired
Minimum typical duty cycle
f_typ(avg) x D_max
ON-time of the power FET controlled by OUT
OFF-time duration required for CrCM operation
Inductance
Transformer turns ratio, Ns/Np
Peak switch current within a switching cycle
User adjustable delay before the next switching
cycle begins
The first calculation required is to determine the required
inductance. The desired inductance can be calculated using
Equation 1.
L_est
=
--------------^V-----^O-----^â‹---⁽---^V----^I--^N----⁽--^r--^m-----^s---⁾---–-----^V----^O-----⁾--------------
2 ⋅ f_min(avg) ⋅ I_O ⋅ 2 ⋅ V_IN(rms)
H
(EQ. 1)
where L_est is the inductance required to achieve the selected
operating frequency at the peak of the AC voltage waveform.
Note that Equation 1 calculates the required inductance when
operating at the DC equivalent input voltage. It does not take into
account the reduction in conduction angle that occurs when the
instantaneous input voltage is less than the output voltage.
Equation 2 corrects for this.
π
–
2
⋅
s
i
n^–1
⎛
⎜
--------------^V-----^O----------------_⎟⎞
L
=
L_e
st
⋅
---------------------------------^⎝-------²-----^â‹---^V----^I--^N----⁽--^r--^m-----^s---⁾--^⎠-
π
H
(EQ. 2)
The maximum ON-time can be found using Equation 3.
t_ON
=
---------²-----^â‹---^I--^O-----^â‹---^L----------
V_IN(rms) – V_O
s
(EQ. 3)
The peak current at the end of the ON-time is shown in
Equation 4:
I_p(peak)
=
-⁽------²-----^â‹---^V----^I--^N----⁽--^r---^m----^s---⁾----–----^V----^O-----⁾---^â‹---^t--^O-----^N--
L
A
(EQ. 4)
And the OFF-time is shown in Equation 5:
t_OFF
=
L_-----⋅_---I_{--p----(--p---e---a----k---)}
V_o
s
(EQ. 5)
The lowest switching frequency is the reciprocal of the sum of the
ON-time, the OFF-time, and the delay time shown by Equation 6.
f_min
=
-------------------------¹--------------------------
t_ON + t_OFF + t_delay
Hz
(EQ. 6)
The delay time can be approximated if the equivalent
drain-source capacitance (C_oss) of the primary switch is known.
This value should also include any parasitic capacitance on the
drain node. These parameters may not be known during the early
stages of the design, but are typically on the order of 300ns to
500ns.
t_d
e
l
a
y
≈
-^π----^â‹--------^L---^p-----^â‹---⁽--^C-----^o---^s---^s----+-----^C----^o----^t--^h---^e---^r---⁾
2
s
(EQ. 7)
If the lowest frequency does not meet the requirements, then
iterative calculations may be required.
The highest frequency is determined by the shortest ON-time
summed with t_delay. The shortest ON-time occurs at high line and
minimum load, and occurs at or near the AC zero crossing when
the primary (and secondary) current is zero. The minimum
ON-time the ISL1903 can produce is ~200ns, suggesting an
operating frequency above 1MHz. Regardless, the maximum
frequency clamp limits the frequency to about 1MHz.
Once the inductance is determined, the general formulae to
calculate the ON-time and OFF-time at an equivalent DC input
voltage and load are:
t_OFF
=
L_-----⋅_---2_-----⋅_---I_--O---
V_O
s
(EQ. 8)
t_ON
=
--^L-----^â‹---²----^â‹---^I--^O----
V_IN(rms)
s
(EQ. 9)
It is clear from the equations that there is a linear relationship
between load current and frequency. At some light load the
frequency will be limited by the maximum frequency clamp.
There is an inverse relationship between the input voltage and
frequency and its effect is restricted by the input voltage range of
the application.
It should be noted, however, that Equations 8 and 9 assume full
conduction angle of the AC mains. There are two issues
regarding actual conduction angle. First, there is no power
delivered to the load until the AC mains instantaneous voltage
exceeds the output voltage. Like any buck converter, the input
voltage must be higher than the output voltage. Secondly, when
conduction angle modulating dimmers are used to block a
portion of each AC half-cycle, the switching currents remain
essentially unchanged during the conduction portion of the AC
half-cycle as the conduction angle is reduced. The conduction
angle is reduced, not the amplitude of the waveform envelope.
The result being the steady state frequency behavior will not vary
much as the conduction angle is reduced depending on the
linearity of the conduction angle and the control loop reference
gain. See Figure 7 on page 12.
Soft-Start Operation
Soft-start is not user adjustable and is fixed at ~ 400ms. Both the
duty cycle and control loop reference are affected by soft-start.
Soft-starting both the duty cycle and the reference ensures a well
behaved closed loop soft-start that results in virtually no
overshoot.
14
FN8285.1
September 20, 2012