+

-

+

-

osc

EAMP

LOGIC

Reset

DriveN

DriveP

+

-

Reset

LIGHT LOAD

COMP

REF

Reset Reset

SET Reset

+

-

Reset

+

-

OVP

REF

PWM

COMP

OVP

COMP

C3

B3

A3

A1

B1

CIN

L

VIN

FB

VIN

COUT

x

FB

CNTRL

SHDN

AGND

GND

VSW

VOUT

+

-

UVP

REF

UVP

COMP

Reset

Dlimit

Duty Limit

Comp

Current

Sense

THERMAL

SHUTDOWN

SHUTDOWN

COMP

Body Diode

Control

+

-

C2

C1

A2

LED

R

LM3501

www.ti.com

SNVS230C – DECEMBER 2003 – REVISED MAY 2013

Operation

Figure 33. LM3501 Block Diagram

The LM3501 utilizes a synchronous Current Mode PWM control scheme to regulate the feedback voltage over

almost all load conditions. The DC/DC controller acts as a controlled current source ideal for white LED

applications. The LM3501 is internally compensated thus eliminating the requirement for any external

compensation components providing a compact overall solution. The operation can best be understood referring

to the block diagram in Figure 33. At the start of each cycle, the oscillator sets the driver logic and turns on the

NMOS power device conducting current through the inductor and turns off the PMOS power device isolating the

output from the VSW pin. The LED current is supplied by the output capacitor when the NMOS power device is

active. During this cycle, the output voltage of the EAMP controls the current through the inductor. This voltage

will increase for larger loads and decrease for smaller loads limiting the peak current in the inductor minimizing

EMI radiation. The EAMP voltage is compared with a voltage ramp and the sensed switch voltage. Once this

voltage reaches the EAMP output voltage, the PWM COMP will then reset the logic turning off the NMOS power

device and turning on the PMOS power device. The inductor current then flows through the PMOS power device

to the white LED load and output capacitor. The inductor current recharges the output capacitor and supplies the

current for the white LED branches. The oscillator then sets the driver logic again repeating the process. The

Duty Limit Comp is always operational preventing the NMOS power switch from being on more than one cycle

and conducting large amounts of current.

The LM3501 has dedicated protection circuitry active during normal operation to protect the IC and the external

components. The Thermal Shutdown circuitry turns off both the NMOS and PMOS power devices when the die

temperature reaches excessive levels. The LM3501 has a UVP Comp that disables both the NMOS and PMOS

power devices when battery voltages are too low preventing an on state of the power devices which could

conduct large amounts of current. The OVP Comp prevents the output voltage from increasing beyond 15.5V

(LM3501-16) and 20.5V (LM3501-21) when the primary white LED network is removed or if there is an LED

failure, allowing the use of small (16V for LM3501-16 and 25V for LM3501-21) ceramic capacitors at the output.

This comparator has hysteresis that will regulate the output voltage between 15.5V and 14.6V typically for the

LM3501-16, and between 20.5V and 19.5V for the LM3501-21. The LM3501 features a shutdown mode that

reduces the supply current to 0.1 uA and isolates the input and output of the converter. The CNTRL pin can be

used to change the white LED current. A CNTRL voltage above 125 mV will enable power to the LEDs and a

voltage lower than 75 mV will turn off the power to the LEDs.

Copyright © 2003–2013, Texas Instruments Incorporated

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