Operation

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 preventing the use of any external

compensation components providing a compact overall so-

lution. The operation can best be understood referring to the

block diagram in Figure 2. 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 V

SW

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 volt-

age. 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 de-

vice. 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 oscil-

lator 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 compo-

nents. 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.

20065304

FIGURE 2. LM3501 Block Diagram

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