LM3402HVMR Datasheet, PDF (17/38 Pages) Texas Instruments – 0.5A Constant Current Buck Regulator for Driving High Power LEDs

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
inductance. D1 must also be rated to handle the average current, I_D, calculated as:
I_D = (1 – D) x I_F
(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
typical diode forward voltage, V_D, from the I-V curve on the product datasheet and then multiplying it by I_D. Diode
datasheets will also provide a typical junction-to-ambient thermal resistance, θ_JA, which can be used to estimate
the operating die temperature of the Schottky. Multiplying the power dissipation (P_D = I_D x V_D) by θ_JA gives the
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.
25
20
15
10
5
0
0
1
2
3
4
5
6
SW VOLTAGE (V)
Figure 23. LED Current From SW Pin
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