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HT7L4091 数据表(PDF) 14 Page - Holtek Semiconductor Inc

部件名 HT7L4091
功能描述  Universal Step-Down PWM Control For High Brightness LED Lighting Control
PDF  26 Pages
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制造商  HOLTEK [Holtek Semiconductor Inc]
网页  http://www.holtek.com
标志 HOLTEK - Holtek Semiconductor Inc

HT7L4091 数据表(HTML) 14 Page - Holtek Semiconductor Inc

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Rev. 1.00
14
���i� ��� �011
Rev. 1.00
15
���i� ��� �011
HT7L4091
Universal Step-Down PWM Control
For High Brightness LED Lighting Control
Off-time Resistor – RT
A resistor connected to the RT pin determines the off-time which has a variation range from -10% to
+10%. Since the working frequency has a minimum target, the CT is considered to calculated the RT:
Toff = CT×RT → RT ≤ 1�.173us
44�F
= 413.03kΩ
Choose 390KΩ and 13KΩ for RT a�e used
The off time is:
Toff_ty�= CT_ty�×RT = 40� x 403K = 16.1us
Toff_max= CT_max×RT = 44� x 403K = 17.7us
Toff_min= CT_max×RT = 36� x 403K = 14.5us
The actua� minimum f�equency can be ca�cu�ated as:
−
−
=
=
=
max
PWM_ min
off_ max
1D
1 0.�731
F
41.17K
T
17.7u
FPWM_TYP@Vac_min= 44.3KHz� FPWM_MAX@Vac_min =50.1 KHz
Inductor Design
The ripple current is selected to be 30% of the nominal LED current. If the LED average current
ILED is 400mA, the LED string Voltage = n × VLED, max = 8 × 3.3V where VLED, max is the LED
maximum forward voltage, then the inductor can be calculated by the following formula.
off
LED�max
LED
T
nV
L
I
Ri���e
××
=
×
=
3
.
0
400
15
.
3
8
7
.
17
mA
us
= 3.717mH
Choose L=3.� mH
Current Sense Resistor – RCS
This current flows through the external sense resistor RCS and produces a ramp voltage on the CS
pin. The comparators are constantly comparing the CS pin voltage with both the voltage on the LD
pin and the internal 250mV reference voltage. Once the blanking time has elapsed, the output of
these comparators can then reset the flip flop. When one output of these two comparators switches
high, the flip flop is reset and the gate drive output switches low. The gate drive output stays low
until the SR flip flop is set by the oscillator. In assuming a 30% ripple in the inductor, the current
sense resistor RCS can be obtained using the following formula:
(
)
CS
�eak
LEDavg
0.�5
0.�5
R
1
I
1
Ri���e I
�
=
=
+×
⋅
=
0.�5
(1 0.5 0.3) 400m�
+×
×
= 0.543Ω
Choose RCS = 0.54Ω



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