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PDF SN3910 Data sheet ( Hoja de datos )
| Número de pieza | SN3910 | |
| Descripción | Universal High Brightness LED Driver | |
| Fabricantes | SI-EN | |
| Logotipo |  |
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Preliminary
SN3910
Universal High Brightness LED Driver
General Description
The SN3910 is an open loop, current mode, control LED
driver IC. The SN3910 operates constant off-time mode. It
allows efficient operation of High Brightness (HB) LEDs
from voltage sources ranging from 8VDC up to 450VDC or
110VAC/220VAC. The SN3910 includes a PWM dimming
input that can accept an external control signal with a duty
ratio of 0 - 100% and a frequency of up to a few kilohertz.
It also includes a 0 - 250mV linear dimming input which
can be used both for linear dimming and temperature
compensation of the LED current.
The SN3910 is ideally suited for buck LED drivers. Since
the SN3910 operates in open loop current mode control, the
controller achieves good output current regulation without
the need for any loop compensation. PWM dimming
response is limited only by the rate of rise and fall of the
inductor current, enabling very fast rise and fall times.
Features
z Switch mode controller for single switch LED drivers
z Open loop peak current controller
z Wide input range from 8VDC to 450VDC or
110VAC/220VAC
z Application from a few mA to more than 1A output
z Up to hundreds of LEDs
z Constant off-time operation
z Linear and PWM dimming capability
z Requires few external components for operation
z Temperature compensation to regulate LED current
z SOP-8 package
Applications
z DC/DC or AC/DC LED driver applications
z RGB backlighting LED driver
z General purpose constant current source
z Signal and decorative LED lighting
Application Circuit
VINDC
8~450V
RIN
CIN
R1
Themistor
NTC
Close to LED
D1
VIN
SN3910
PWMD
GATE
VREF
L1
Q1
LD
TOFF
CS
GND
RCS
COFF
CO
Jul 2008 Ver0.1
1 SI-EN Technology
1 page  
Preliminary
SN3910
Application Information
The SN3910 is optimized to drive buck LED drivers using
open-loop peak current mode control. This method of
control enables fairly accurate LED current control without
the need for high side current sensing or the design of any
closed loop controllers. The IC uses very few external
components and enables both Linear and PWM dimming of
the LED current.
A capacitor connected to the Toff pin programs the off-time.
The oscillator produces pulses at regular intervals. These
pulses set the SR flip-flop in the SN3910 which causes the
GATE driver to turn on. When the FET turns on, the
current through the inductor starts ramping up. This current
flows through the external sense resistor RCS and produces
a ramp voltage at the CS pin. The comparators are
constantly comparing the CS pin voltage to both the voltage
at the LD pin and the internal 250mV. Once the blanking
timer is complete, the output of these comparators is
allowed to reset the flip flop. When the output of either one
of the two comparators goes high, the flip flop is reset and
the GATE output goes low. The GATE goes low until the
SR flip flop is set by the oscillator. Assuming a 30% ripple
in the inductor, the current sense resistor RCS can be set
using:
RCS
=
0.25V(orVLD)
1.15 • ILED(A)
A constant off-time peak current control scheme can easily
operate at duty cycles greater then 0.5 and also gives
inherent input voltage rejection making the LED current
almost insensitive to input voltage variations.
Input Voltage Regulator
The current sense input of the SN3910 goes to the
noninverting inputs of two comparators. The inverting
terminal of one comparator is tied to an internal 250mV
reference whereas the inverting terminal of the other
comparator is connected to the LD pin. The outputs of both
these comparators are fed into an OR GATE and the output
of the OR GATE is fed into the reset pin of the flip-flop.
Thus, the comparator which has the lowest voltage at the
inverting terminal determines when the GATE output is
turned off.
The outputs of the comparators also include a 150-280ns
blanking time which prevents spurious turn-offs of the
external FET due to the turn-on spike normally present in
peak current mode control. In rare cases, this internal
blanking might not be enough to filter out the turn-on spike.
In these cases, an external RC filter needs to be added
between the external sense resistor (RCS) and the CS pin.
Please note that the comparators are fast (with a typical
80ns response time). A proper layout minimizing external
inductances will prevent false triggering of these
comparators.
Oscillator
The oscillator in the SN3910 is controlled by a single
capacitor connected at the Toff pin. The equation governing
the Toff-time of oscillation period is given by:
TOFF _ TIME
=
0.51×10
−6
×
(1
+
COFF
10 pF
)
Linear Dimming
The Linear Dimming pin is used to control the LED current.
There are two cases when it may be necessary to use the
Linear Dimming pin.
When a voltage is applied at the decent resistor, the
SN3910 maintains a constant 7.5V at the VIN pin. This
voltage is used to power the IC and any external resistor
dividers needed to control the IC. The VIN pin must be
bypassed by a low ESR capacitor to provide a low
impedance path for the high frequency current of the output
GATE driver.
The input current drawn from the VIN pin is a sum of the
1.0mA current drawn by the internal circuit and the current
drawn by the GATE driver (which in turn depends on the
switching frequency and the GATE charge of the external
FET).
IIN ≈ 1.0mA + QG • fS
In the above equation, fS is the switching frequency and QG
is the GATE charge of the external FET (which can be
obtained from the datasheet of the FET).
Current Sense
► In some cases, it may not be possible to find the exact
RCS value required to obtain the LED current when the
internal 250mV is used. In these cases, an external voltage
divider from the VDD pin can be connected to the LD pin
to obtain a voltage (less than 250mV) corresponding to the
desired voltage across RCS.
► Linear dimming may be desired to adjust the current
level to reduce the intensity of the LEDs. In these cases, an
external 0-250mV voltage can be connected to the LD pin
to adjust the LED current during operation.
To use the internal 250mV, the LD pin can be connected to
VDD.
PWM Dimming
PWM Dimming can be achieved by driving the PWMD pin
with a low frequency square wave signal. When the PWM
signal is zero, the GATE driver is turned off and when the
PWMD signal if high, the GATE driver is enabled. Since
Jul 2008 Ver0.1
5 SI-EN Technology
5 Page | ||
| Páginas | Total 7 Páginas | |
| PDF Descargar | [ Datasheet SN3910.PDF ] | |
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