LT3477 datasheet(14/26 Pages) AD

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LT3950

Rev. 0

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The PWM dimming generator clock offers two program-

ming options. For a separate, free-running clock with

respect to the switching frequency, connect a resistor

from the RP pin to ground. Similar to the RT pin, the RP

pin sets the frequency of PWM dimming as a function of

resistance.

Table 2. Selected RP Values and PWM Dimming Frequency

RP Value (kΩ)

PWM Dimming Frequency (Hz)

634

100

274

200

162

300

107

400

500

Alternatively, the PWM dimming clock can lock to the

switching clock. To do this, simply tie the RP pin to

INTVCC. In this mode, the PWM clock will be the switch-

ing clock divided by 4096.

Pulse Width Modulation (PWM) Dimming

Pulse width modulation (PWM) allows high dynamic

range dimming of the LED load. When using PWM dim-

ming, a pulse train with duty ratio proportional to desired

LED current controls the load. During ON periods, the

part operates normally. During OFF periods the part stops

switching. While the part is not switching, the compensa-

tion node is high impedance to minimize changes to the

compensation capacitor voltage. This reduces transient

settling time when the next ON period arrives. In addi-

tion to this, LT3950 provides an optional load disconnect.

Disconnecting the load makes turn-off much faster, as the

output capacitor does not continue to conduct current

into the load. Transient settling time is also shorter when

turning back on, as the output capacitor’s state is less

affected by the load.

To use the external load disconnect, tie a PMOS in series

with the load such that the source of the PMOS connects

to the ISN node, and the drain to the LED load. Connect

the gate of the PMOS to the PWMTG pin. The voltage at

the PWMTG pin will vary between VISP and VISP – 7.5V

to turn the PMOS off and on. Note that this configuration

works for any of the supported power stage topologies.

APPLICATIONS INFORMATION

For more information on power stage topologies, see the

example application circuits.

The PWM pin allows two modes of PWM dimming. The

first mode is external PWM. In this mode, a digital signal

created by some other device, such as a microprocessor,

drives the PWM pin. This PWM signal directly controls

the part: when this signal is high, the part runs, when this

signal is low, the part does not run, and disconnects the

load if an external PMOS is used. Tying the PWM pin to

INTVCC results in continuous, uninterrupted operation.

Conversely, tying the PWM pin to ground results in the

system remaining idle indefinitely. For ON time < 1µs,

use a low QG (<10nC) MOSFET and low or zero value for

RC at VC pin. External PWM supports dimming dynamic

range up to 20,000:1.

The second mode of PWM dimming is internal. When

using internal PWM dimming, the analog voltage at the

PWM pin controls the duty ratio of the PWMTG signal. The

voltage range for internal PWM dimming is from 0.2V to

1.2V at the PWM pin. The internal PWM generator con-

verts the voltage at the PWM pin to a 7-bit digital rep-

resentation. The analog to digital converter responsible

for this uses a linear scale, each code is around 7.8mV

wide. Each 7-bit code corresponds to a unique duty ratio

value. The values of duty ratio are separated exponentially.

Another way to phrase it is that each time the 7-bit code

increases in value by 1, the duty ratio is multiplied by a

constant scaling factor. A log-scale plot of duty ratio vs

PWM pin voltage appears below.

Figure 4. PWM Duty Ratio

PWM PIN VOLTAGE (V)

0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2

0.5

100

3950 F04

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