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SW VOLTAGE (V)

LM3402, LM3402HV

www.ti.com

SNVS450E – SEPTEMBER 2006 – REVISED MAY 2013

Ceramic capacitors are the best choice for the input to the LM3402/02HV due to their high ripple current rating,

low ESR, low cost, and small size compared to other types. When selecting a ceramic capacitor, special

attention must be paid to the operating conditions of the application. Ceramic capacitors can lose one-half or

more of their capacitance at their rated DC voltage bias and also lose capacitance with extremes in temperature.

A DC voltage rating equal to twice the expected maximum input voltage is recommended. In addition, the

minimum quality dielectric which is suitable for switching power supply inputs is X5R, while X7R or better is

preferred.

RECIRCULATING DIODE

The LM3402/02HV is a non-synchronous buck regulator that requires a recirculating diode D1 (see the Typical

Application circuit) to carrying the inductor current during the MOSFET off-time. The most efficient choice for D1

is a Schottky diode due to low forward drop and near-zero reverse recovery time. D1 must be rated to handle the

maximum input voltage plus any switching node ringing when the MOSFET is on. In practice all switching

converters have some ringing at the switching node due to the diode parasitic capacitance and the lead

(15)

This calculation should be done at the maximum expected input voltage. The overall converter efficiency

becomes more dependent on the selection of D1 at low duty cycles, where the recirculating diode carries the

load current for an increasing percentage of the time. This power dissipation can be calculated by checking the

datasheets will also provide a typical junction-to-ambient thermal resistance,

temperature rise. The diode case size can then be selected to maintain the Schottky diode temperature below

the operational maximum.

LED CURRENT DURING DIM MODE

The LM3402 contains high speed MOSFET gate drive circuitry that switches the main internal power MOSFET

between “on” and “off” states. This circuitry uses current derived from the VCC regulator to charge the MOSFET

during turn-on, then dumps current from the MOSFET gate to the source (the SW pin) during turn-off. As shown

in the block diagram, the MOSFET drive circuitry contains a gate drive under-voltage lockout (UVLO) circuit that

ensures the MOSFET remains off when there is inadequate VCC voltage for proper operation of the driver. This

watchdog circuitry is always running including during DIM and shutdown modes, and supplies a small amount of

current from VCC to SW. Because the SW pin is connected directly to the LEDs through the buck inductor, this

current returns to ground through the LEDs. The amount of current sourced is a function of the SW voltage, as

shown in Figure 23.

Figure 23. LED Current From SW Pin

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