How to provide variable duty circle for LED brightness contr

[montgemory]

variable led brightness

I am studying the DC-DC converter and using it to drive a LED.
Now I am designing a current mode PWM which control the switch at constant frequency and variable duty circle.
I look up some IC's datasheets and I'd like to design it just like the LM3410 of National Semiconductor or TPS61042 of TI.
But I don't know how can use the PWM to provide the variable duty circle for LED brightness control.
Please explain the following sentences detailedly,
1. "The LM3410 supplies a regulated LED current by switching the internal NMOS control switch at constant frequency and variable duty cycle."(the last 3 lines on the left half page 8 of the LM3410)
2. the second paragraph on page 10 of the TPS61042
Please tell me how can it be realized?
I appreciate all your help.

 

[fragment]

ledbrightness.c

These two IC use different approaches for PWM.
1. TPS61042 enables external PWM. Let we set LED current to I0 value without using PWM.
Now we apply signal to CTRL pin with frequency 100 Hz and duty cycle 50%. It means that half time IC will be disabled and LED current will be I0/2. Changing duty cycle we can change current through LEDs.
2. LM3410 uses internal PWM to keep voltage on FB pin 190 mV. If feedback voltage begin to increase IC decrease duty cycle and vice versa.