LM3402 datasheet(12/39 Pages) TI1

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TSW

300 ns

VO(MIN) = VIN x

TSW

TSW - 300 ns

VO(MAX) = VIN x

TSW = 1/fSW

nMAX =

VF(MAX)

VO(max) - 200 mV

LM3402, LM3402HV

SNVS450F – SEPTEMBER 2006 – REVISED OCTOBER 2015

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Feature Description (continued)

The maximum number of LEDs, nMAX, that can be placed in a single series string is governed by VO(MAX) and the

maximum forward voltage of the LEDs used, VF(MAX), using the expression:

(7)

At low switching frequency the maximum duty cycle and output voltage are higher, allowing the LM3402/HV to

regulate output voltages that are nearly equal to input voltage. The following equation relates switching frequency

to maximum output voltage.

(8)

7.3.4 Minimum Output Voltage

The minimum recommended ON-time for the LM3402/HV is 300 ns. This lower limit for tON determines the

minimum duty cycle and output voltage that can be regulated based on input voltage and switching frequency.

The relationship is determined by the following equation:

(9)

7.3.5 High Voltage Bias Regulator

The LM3402/HV contains an internal linear regulator with a 7-V output, connected between the VIN and the VCC

pins. The VCC pin should be bypassed to the GND pin with a 0.1-µF ceramic capacitor connected as close as

possible to the pins of the device. VCC tracks VIN until VIN reaches 8.8 V (typical) and then regulates at 7 V as

VIN increases. Operation begins when VCC crosses 5.25 V.

7.3.6 Internal MOSFET and Driver

The LM3402/HV features an internal power MOSFET as well as a floating driver connected from the SW pin to

the BOOT pin. Both rise time and fall time are 20 ns each (typical) and the approximate gate charge is 3 nC. The

high-side rail for the driver circuitry uses a bootstrap circuit consisting of an internal high-voltage diode and an

external 10 nF capacitor, CB. VCC charges CB through the internal diode while the power MOSFET is off. When

the MOSFET turns on, the internal diode reverse biases. This creates a floating supply equal to the VCC voltage

minus the diode drop to drive the MOSFET when its source voltage is equal to VIN.

7.3.7 Fast Shutdown for PWM Dimming

The DIM pin of the LM3402/HV is a TTL logic compatible input for low-frequency PWM dimming of the LED. A

logic low (less than 0.8V) at DIM will disable the internal MOSFET and shut off the current flow to the LED array.

While the DIM pin is in a logic low state the support circuitry (driver, bandgap, VCC) remains active to minimize

the time needed to turn the LED array back on when the DIM pin sees a logic high (above 2.2 V). A 75 µA

(typical) pullup current ensures that the LM3402/HV is on when DIM pin is open circuited, eliminating the need

for a pullup resistor. Dimming frequency, fDIM, and duty cycle, DDIM, are limited by the LED current rise time and

fall time and the delay from activation of the DIM pin to the response of the internal power MOSFET. In general,

fDIM should be at least one order of magnitude lower than the steady state switching frequency to prevent

aliasing.

7.3.8 Peak Current Limit

The current limit comparator of the LM3402/HV will engage whenever the power MOSFET current (equal to the

inductor current while the MOSFET is on) exceeds 735 mA (typical). The power MOSFET is disabled for a cool-

down time that is 10x the steady-state ON-time. At the conclusion of this cool-down time the system re-starts. If

the current limit condition persists the cycle of cool-down time and restarting will continue, creating a low-power

hiccup mode, minimizing thermal stress on the LM3402/HV and the external circuit components.

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