LT3477EUF#TRPBF Linear Technology, LT3477EUF#TRPBF Datasheet - Page 11

• Manufacturer Part Number: LT3477EUF#TRPBF
• Description: IC LED DRVR HP CONST CURR 20-QFN
• Manufacturer: Linear Technology
• Type: High Power, Constant Currentr

Specifications of LT3477EUF#TRPBF

• Constant Current: Yes
• Constant Voltage: Yes
• Topology: Flyback, PWM, SEPIC, Step-Down (Buck), Step-Up (Boost)
• Number Of Outputs: 1
• Internal Driver: Yes
• Type - Primary: General Purpose
• Type - Secondary: White LED
• Frequency: 200kHz ~ 3.5MHz
• Voltage - Supply: 2.5 V ~ 25 V
• Mounting Type: Surface Mount
• Package / Case: 20-QFN
• Operating Temperature: -40°C ~ 85°C
• Current - Output / Channel: 3A
• Internal Switch(s): Yes
• Efficiency: 93%
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Voltage - Output: -

APPLICATIONS INFORMATION
PWM Dimming
For LED applications where a wide dimming range is
required, two competing methods are available: analog
dimming and PWM dimming. The easiest method is to
simply vary the DC current through the LED—analog
dimming—but changing LED current also changes its
chromaticity, undesirable in many applications. The bet-
ter method is PWM dimming, which switches the LED
on and off, using the duty cycle to control the average
current. PWM dimming offers several advantages over
analog dimming and is the method preferred by LED
manufacturers. By modulating the duty cycle of the PWM
signal, the average LED current changes proportionally
as illustrated in Figure 5. The chromaticity of the LEDs
remains unchanged in this scheme since the LED current is
either zero or at programmed current. Another advantage
of PWM dimming over analog dimming is that a wider
dimming range is possible.
The LT3477 is a DC/DC converter that is ideally suited for
LED applications. For the LT3477, analog dimming offers
a dimming ratio of about 10:1; whereas, PWM dimming
with the addition of a few external components results in
a wider dimming range of 500:1. The technique requires a
PWM logic signal applied to the gate of both NMOS (refer
to Figure 7). When the PWM signal is taken high the part
runs in normal operation and I
0.01
100
0.1
10
Figure 5. LED Current vs PWM Duty Cycle
Wide Dimming Range (500:1)
1
0.1
R
V
BOOST
4 LEDs
PWM FREQUENCY = 100Hz
T
IN
= 6.81k
= 5V
PWM DUTY CYCLE (%)
1
LED
10
= 100mV/R
3477 F05
100
SENSE
runs
through the LEDs. When the PWM input is taken low, the
LEDs are disconnected and turn off. This unique external
circuitry produces a fast rise time for the LED current,
resulting in a wide dimming range of 500:1 at a PWM
frequency of 100Hz.
The LED current can be controlled by feeding a PWM signal
with a broad range of frequencies. Dimming below 80Hz is
possible, but not desirable, due to perceptible fl ashing of
LEDs at lower PWM frequencies. The LED current can be
controlled at higher frequencies, but the dimming range
decreases with increasing PWM frequency, as seen in
Figure 6.
PWM dimming can be used in boost (shown in Figure 7), buck
mode (shown in Figure 8) and buck-boost mode (shown
in Figure 9). For the typical boost topology, effi ciency ex-
ceeds 80%. Buck mode can be used to increase the power
handling capability for higher current LED applications. A
buck-boost LED driver works best in applications where
the input voltage fl uctuates to higher or lower than the
total LED voltage drop.
In high temperature applications, the leakage of the
Schottky diode D1 increases, which in turn, discharges the
output capacitor during the PWM off time. This results in
a smaller effective LED dimming ratio. Consequently, the
dimming range decreases to about 200:1 at 85°C.
Figure 6. Dimming Range vs PWM Frequency
1000
100
10
1
0.1
R
T
= 6.81k
PWM FREQUENCY (kHz)
1
10
3477 F06
LT3477
100
11
3477fc