IS31LT3948-GRLS2-TR ISSI, IS31LT3948-GRLS2-TR Datasheet - Page 6

• Manufacturer Part Number: IS31LT3948-GRLS2-TR
• Description: LED Lighting Drivers PFM Mode Boost LED Drvr
• Manufacturer: ISSI

Specifications of IS31LT3948-GRLS2-TR

• Input Voltage: 5 V to 100 V
• Maximum Supply Current: 400 uA
• Maximum Operating Temperature: + 85 C
• Mounting Style: SMD/SMT
• Package / Case: SOP-8
• Factory Pack Quantity: 2500

IS31LT3948
Application Information
Internal 5V Regulator
The IS31LT3948 includes an internal shunt regulator of
5V (typical) connected to the V
voltage is higher than 5V, connect V
appropriately valued, current limiting resistor. The
regulator maintains a 5V power supply for the internal
NMOS switch gate driver and the internal control
circuitry. In applications where the input voltage is 5V,
connect the input voltage directly to V
connected directly to VIN, VIN may not exceed 5V.
Bypass the V
(recommended 10µF ceramic capacitor) to provide a
high frequency path to GND.
The current required by IS31LT3948 is 0.25mA (typical)
plus the switching current of the external switch. The
switching frequency of the external NMOS affects the
amount of current required, as does the NMOS’s gate
charge requirement (found on the NMOS data sheet).
Where f
external NMOS gate charge.
Under Voltage Lockout
IS31LT3948 features an under voltage lockout
threshold of 2.7V (typical) with a hysteresis of 300mV.
The chip is disabled when V
enabled when V
Step-up Converter
IS31LT3948’s step-up converter uses a peak current
mode topology wherein the CS pin voltage determines
the peak current in the inductor of the converter and
hence the duty cycle of the GATE switching waveform.
The basic loop uses a pulse from an internal oscillator
to set an RS flip-flop and turn on the external power
NMOS. After the blanking time, the inductor current is
sensed during the GATE on period by a sense resistor,
R
current increases in the NMOS and inductor until the
voltage across the sense resistor reaches the CS
threshold, at which time NMOS is turned off. Once the
NMOS is turned off, the inductor reverses polarity,
providing the voltage boost, and the current of inductor
will decrease until the FB pin voltage drops below
internal reference voltage and the NMOS is then
turned on again. This operation repeats each cycle.
Note: In the case where the FB pin voltage does not
exceed the FB reference voltage of 0.3V, such as at
start-up, the NMOS will remain off for the programmed
minimum TOFF time, then the NMOS is switched on
again.
Integrated Silicon Solution, Inc. – www.issi.com
Rev.B, 12/24/2012
CS
, in the source of the external power NMOS. The
S
is the switching frequency and Q
I
IN
CC
 25
pin using a low ESR capacitor
CC
0
.
exceeds 2.7V.
mA

Q
CC
G
CC
is lower than 2.4V and

pin. When the input
f
S
CC
CC
to VIN using an
. When V
(1)
G
is the
CC
is
LED Current Control
IS31LT3948 regulates the LED current by sensing the
voltage across an external sense resistor in series with
the LEDs. The voltage is sensed via the FB pin where
the internal feedback reference voltage is 0.3V(typical).
The LED current can be set from following equation
easily.
In order to have an accurate LED current, precision
resistors are required (1% is recommended).
Setting the Over Voltage Protection
The open string protection is achieved through the over
voltage protection (OVP). In some cases, an LED
string failure results in a feedback voltage that is
always zero. If this happens, the part then keeps
boosting the output voltage higher and higher. If the
output voltage reaches the programmed OVP
threshold, the protection will be triggered and stop the
switching action. To make sure that the circuit
functions properly, the OVP setting resistor divider
must be set with an appropriate value. The
recommended
(choose the larger one) higher than the output voltage
for normal operation.
Where, V
OVP level.
Dimming Control
There are two methods for dimming.
1) External NMOS PWM dimming:
When the PWM input is high (V
IS31LT3948 operates normally to regulate the output
current. When PWM is low logic (V
and IS31LT3948 is shutdown. Using a fixed frequency
PWM signal and changing the duty cycle adjusts the
average output current. The recommended 5V PWM
frequency is between 200Hz and 1KHz. M2 is
recommended to use AP2306.
OVP-TH
V
OVP
is 1V, and V
V

IS31LT3948
OVP
V
I
OVP
out
point is about 1.2 times or 5V


Figure 10
TH
0
R

3 .
fb
OVP
R
4
R
is the output voltage
H

>2.4V), M2 is on and
5
R
(2)
L
5
<0.5V), M2 is off
(3)
6