LT3952 datasheet(24/32 Pages) LINER

24 / 32 page

LT3952

3952fb

For more information www.linear.com/LT3952

OPERATION

three-stating the SS pin. This is to avoid sinking quiescent

current in the case that an external resistor pull-up is

connected to SS.

When the device leaves shutdown mode, the NMOS pull-

downswitchisactivatedtoclearthevoltageatthesoft-start

capacitor. The device waits in this state until the voltage

at SS drops below 0.2V, and until start-up is enabled

as defined by PWM > 1.2V and CTRL > 100mV. If both

enable signals are already valid upon leaving shutdown

mode, the NMOS pull-down is activated for 10µs longer

than is required to pull the SS voltage below 0.2V. In all

cases, this action provides a start-up profile where the

initial voltage at SS can be considered approximately 0V.

In the event that the device is shut down by EN/UVLO

or the INTVCC voltage dropping below 2.68V, the SS pin

initially goes high impedance and then restarts the clear,

charge sequence as described.

Start-Up Mode: Once start-up is enabled by PWM and

CTRL both valid, the NMOS pull-down switch is disabled

and the 25A charging current is enabled. The voltage at

SS begins ramping in a linear manner until it reaches 0.2V,

at which point switching and the TG driver are enabled.

From 0.2V to 1.7V on SS, the frequency and the current

limit are increased linearly with SS voltage to provide

a smooth start-up profile with low inrush current. The

required soft-start capacitor for a desired start-up time

is computed from the 1.5V range of SS and the 25A

charge current.

CSS (nF) = 16.67 • tSS (ms)

For example, a soft-start time of roughly 0.5ms is gener-

ated with an 8.2nF capacitor.

Depending on output and load conditions the device may

enter regulation before SS reaches 1.7V, however it is

the combination of the 1.7V threshold and the sensing of

at least one-tenth of the output current defined by CTRL

that signifies the successful completion of soft-start. This

becomes important when start-up is enabled by a low duty

cycle PWM signal. The explanation is as follows:

A low duty cycle PWM signal could cause excessive

start-up times if it were allowed to interrupt the soft-

start sequence. Therefore, once start-up is initiated by

PWM > 1.2V and CTRL > 150mV, it temporarily ignores

a logical disable by either of those signals. The device

continues to soft-start with switching and TG enabled

until either the voltage at SS reaches the 1.7V level, or the

output current reaches one-tenth of the full-scale value. At

this point the device begins following the dimming control

as designated by PWM or CTRL. If at any time an output

overcurrent is detected, SW and TG will be disabled even

as SS continues to charge. This will be discussed in further

detail in the Fault Handling section.

PWM

2V/DIV

20V/DIV

ILED

200mA/DIV

3952 F15

NORMAL

PWM CYCLE

OUTPUT READY:

LATCHED

STARTUP ENDS

LATCHED

STARTUP BEGINS

10ms/DIV

Figure 14. PWM Latched Startup

One note is when PWM dimming using CTRL, the output

current is commanded to zero during the low time of CTRL

even as the soft-start voltage rises.

FaultHandling:Althoughthefaulthandlingsequencemay

change based on the soft-start conditions, the switching

is disabled and the TG driver immediately pulls high upon

detection of an output overcurrent fault. This provides

safe output disconnect even in the case of a dead short

on the output.

When an overcurrent is detected and TG and SW are

disabled, SS is still required to charge to the 1.7V upper

threshold before it begins discharging back down using

the 2.5A current source. If SS is already at or above 1.7V,

it begins discharging immediately upon the disable of TG

and SW. If SS is still in the start-up phase when an output

overcurrent fault is detected, then SS continues charging

even as TG and SW are disabled. It charges to the 1.7V

level before reversing direction and discharging to the

0.2V level with the 2.5A current source. This provides

additional delay to allow the system to recover from the

overcurrent condition.

LT3952 Datasheet(PDF) 24 Page - Linear Technology

LT3952 Datasheet(HTML) 24 Page - Linear Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32

Datasheet Download

Link URL