lt3465a Linear Technology Corporation, lt3465a Datasheet - Page 11

• Manufacturer Part Number: lt3465a
• Description: 2.4mhz White Led Step-up Converters With Built-in Schottky In Thinsot
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
case. The Average FB Voltage vs PWM Duty Cycle curves
of different PWM frequencies with different output ca-
pacitors are shown in Figures 7c and 7d respectively. For
PWM frequency lower than 1kHz, the curves are almost
linear. For PWM frequency higher than 10kHz, the curves
show strong nonlinearity. Since the cause of the
nonlinearity is the output capacitor charging/discharg-
ing, the output capacitance and output voltage also affect
Figure 7c. V
200
180
160
140
120
100
80
60
40
20
Figure 7e.V
0
200
180
160
140
120
100
80
60
40
20
0
0
C
4 LEDs
0
OUT
U
10kHz PWM
C
OUT
= 0.22µF
20
CTRL PWM DUTY CYCLE (%)
= 0.22µF
FB
20
CTRL PWM DUTY CYCLE (%)
FB
U
vs CTRL PWM Duty Cycle
vs CTRL PWM Duty Cycle
40
40
60
W
60
80
80
10Hz
100Hz
1kHz
10kHz
30kHz
3465A F07c
2 LEDs
3 LEDs
4 LEDs
3465A F07e
100
U
100
the nonlinearity in the high PWM frequencies. Because
smaller capacitance corresponds to shorter capacitor
charging/discharging time, the smaller output capaci-
tance has better linearity as shown in Figures 7c and 7d.
Figures 7e and 7f show the output voltage’s effect to the
curves. The PWM signal should be at least 1.8V in
magnitude; lower voltage will lower the feedback voltage
as shown in Equation 1.
Figure 7f.V
200
180
160
140
120
100
200
180
160
140
120
100
80
60
40
20
80
60
40
20
0
Figure 7d. V
0
0
0
30kHz PWM
C
C
4 LEDs
OUT
OUT
10
= 0.22µF
= 0.47µF
20
CTRL PWM DUTY CYCLE (%)
20
FB
CTRL PWM DUTY CYCLE (%)
vs CTRL PWM Duty Cycle
30
FB
40
40
vs CTRL PWM Duty Cycle
LT3465/LT3465A
50
60
60
70
80
80
2 LEDs
3 LEDs
4 LEDs
3465A F07f
10Hz
100Hz
1kHz
10kHz
30kHz
3465A F07d
90
100
100
11
3465afa