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LT3952

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For more information www.linear.com/LT3952

OPERATION

The LT3952 is a fixed-frequency, current mode boost con-

verter with a feature set ideal for driving high power LED

lamps. It provides input and output current regulation, as

well as output voltage regulation and fault handling. The

operation of the LT3952 is best understood by referring to

the Block Diagram. In normal operation, with the PWM pin

low, the switching is disabled, the TG pin is pulled high to

ISP to turn off the PMOS disconnect switch, the VC pin is

high impedance to store the previous switching state on

the external compensation capacitor, and the ISP/ISN pin

bias currents are reduced to leakage levels.

When the PWM pin transitions high, the TG pin transitions

low after a short delay. At the same time, the internal os-

cillator wakes up and generates a pulse to set the PWM

latch, turning on the internal NMOS power switch. The

switch current is sensed and added to a stabilizing slope

compensation ramp, and the resultant signal is compared

to the VC voltage. The inductor current increases linearly

during the switch on-time. When the current sense signal

exceeds the VC value, the latch is reset and the internal

NMOS power switch is turned off. During the switch

off-time, energy is transferred from the inductor and the

inductor current decreases. At the completion of each

oscillator cycle, internal signals such as slope compensa-

tion return to their starting points and a new cycle begins

with the set pulse from the oscillator.

Through this repetitive action, the IC establishes a switch-

ing duty cycle to regulate a current or voltage in the load.

The VC signal is integrated over many switching cycles,

and is an amplified version of the difference between the

LED current sense voltage, measured between ISP and

ISN, and the target difference voltage set by the CTRL pin.

In this manner, the error amplifier sets the correct peak

switch current level to keep the LED current in regulation.

If the error amplifier output increases, more current is

demanded in the switch; if it decreases, less current is

demanded. If the switch current exceeds the internal 4.5A

current limit, the latch is reset regardless of the state of

the PWM comparator. Likewise, in any fault condition; i.e.,

FB overvoltage (VFB > 1.23V), LED overcurrent (VISP-ISN

> 375mV), overvoltage lockout or INTVCC undervoltage

the switching is immediately disabled.

In addition to the VISP-ISN to CTRL feedback loop, the

LT3952 also provides additional loops to control input

current and output voltage. The loops are connected in a

wired-OR configuration, where the auxiliary loops are only

allowed to reduce the value of VC in the event that one or

more exceeds its threshold. This means that even if one

or more of the auxiliary loops is below its regulation point,

VC will not rise unless the output (LED) current sense is

also below its regulation point.

The first auxiliary loop is the output voltage feedback loop

using the FB pin. It is typically used to prevent the output

voltage from exceeding a safe value. The voltage at the FB

pin is compared to an internal reference voltage of 1.2V,

and the amplified difference pulls down the VC pin in the

case that FB exceeds 1.2V. As VC drops, the switching

current reduces and in this manner the output voltage is

regulated so that FB = 1.2V.

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