Trouble with LCDI2C4Bit.h did I burn sth?

Hi,

I have one problem with LCDI2C4Bit.h library.
I use pin6 of my PCF8574 to controll the backlight of LCDdisplay. I use NPN transistor - so when I send lcd.backLight(true) - the backlight should be OFF, when lcd.backLight(false) - ON.

It works great when I send lcd.backLight(false).
When i send lcd.backLight(true) - the backlight blinks! The frequency is like delay function.
I think that earlier was everythink OK, is it possible that i burned sth? The LCD display works good, just the backlight!
I tested it with 2 other PCF8574 and still the same.

My code:

#include <Wire.h>
#include <LCDI2C4Bit.h>

/*
;Connect the following pins from PCF8574 to LCD
;
;P0 - D4
;P1 - D5
;P2 - D6
;P3 - D7
;P4 - RS
;P5 - RW (not used, set to 0 to ground for write)
;P6 - Bl (backlight switch)
;P7 - E1;
*/

LCDI2C4Bit lcd = LCDI2C4Bit(B0111111,2,16);

void setup()
{
  Serial.begin(9600);
  Wire.begin();
  lcd.init();
  lcd.backLight(true); //I send just once to turn the backlight OFF
}

void loop()
{
  lcd.clear();  
  lcd.printIn("Hello World");  
  delay(500);   
}

Any ideas?