AP1601 Datasheet, PDF (9/22 Pages) Anachip Corp – Step-Up DC/DC Converter

OUT
tON
tO F F⁽1^{A )}tO F F 2
Ts
(B) (C)
ï¼»AP1601ï¼½
VDS
tON
tOFF1
tOFF2
OUT
ON
OFF
OFF
ILp
D5
OFF
ON
OFF
ILp
V _{in  t}
L_p
0
0
ILs
ILs
0
I _{Lp _ Peak  VF  VFD5  t}
N
L_S
0
Figure 5. DCM flyback mode wave form
The AP1601 fixed frequency mode is using the T1 auxiliary winding, waiting until the Q1 drain voltage falls off
after the secondary current becomes zero, then moves to the next cycle. Approximations of “tON, tOFF1” time
requirement in Figure 5 are;
t_ON
 ^Lp
V_in
 I _{LP _ Peak}
t_{OFF 1}

I _{LP _ Peak}
N

V_F
L_S
 V_FD5

V_F
L_P
 V_FD5
 I _{LP _ Peak}
N
Next, the tOFF2 period is the time for compensation to the LED VF variation. By detecting the bottom voltage using
the VTHZC threshold, tOFF2 (for example) is defined by (B) or (C) if each switching is observed. However, the
internal CLK operates at a constant cycle while VVF is maintained constant, and tOFF2 will be an ideal period if
averaged for a sufficiently long time (e.g., 10ms for 100kHz switching frequency).
The shortest amount of the tOFF2 time is between (A) and (B) shown in Figure 5. This is the reason why the drain
voltage doesn’t fall off immediately due to resonance caused by the primary side inductance (Lp) of the
transformer and a parasitic capacitor (Clump) between Q1 drain and GND.
Approximation of tOFF2’s minimum time is as follows.
t_{OFF 2 _ min}    L_p  C_lump
If the compensation for LED VF variation is Corr%, the approximation of the set value is as follows.
t_{OFF 2}  (t_ON  t_OFF1  t_{OFF 2_ min} )  Corr%
e.g. When Vin=100V, Lp=680uH, ILp=0.7A, VF=30V, VFD5=0.7V, N=0.4, Clump=100pF and ±Corr%=±30%,
tON=4.76us, tOFF1=6.2us, tOFF2_min=0.82us, and tOFF2 is calculated to be 3.53us.
From this, when the set frequency is approximately 69kHz and energy transmission efficiency of the transformer
η is 0.9, the LED average current will be approximately 337mA.
When the frequency needs to be higher without affecting to the other parameters, tOFF1 should be shorter by
changing the winding ratio (N) lower.
However, when the winding ratio (N) decreases, transformer transmission efficiency tends to be less, and the
AP1601 shortest tOFF1 time is designed at 2us, therefore, please do not be shorter than that in steady state.
In actual AC power source input, Vin is the absolute value of the sine wave, peak current is proportional to Vin
until a certain voltage level, please calculate using the time of the half cycle of the AC power frequency
(50Hz/60Hz).
MS 1454-E-01
9
2012/08