IS31LT3952 IS32LT3952 DEMO BOARD GUIDE DESCRIPTION QUICK START The IS31LT3952 IS32LT3952 is a DC-to-DC switching converter, which integrate an N-channel MOSFET to operate in a buck configuration. The device supply a wide input voltage between 4.5V and 38V and provides a constant current of up to 1.5A for driving a single LED or multiple series connected LEDs. The external resistor, R , is used to adjust LED SET output current, which allowing the output voltage to be automatically adjusted for a variety of LED configurations. The IS31LT3952 IS32LT3952 operates in a fixed Figure 1: Photo of IS31LT3952 IS32LT3952 Evaluation Board frequency mode during switching. There is an external resistor connected between the VCC and TON pins used to configure the on-time (switching frequency). RECOMMENDED EQUIPMENT The switching frequency is dithered for spread spectrum feature to spread the electromagnetic 38VDC power supply emitting energy into a wider frequency band. It is 1 pcs of LED panel (3W LEDs, 2LEDs in parallel helpful to optimize the EMI performance. and then 10 LEDs in series on each panel) A logic input PWM signal to the enable (EN) pin is Multi-meter applied to adjust the LED current. The brightness of LED is proportional to the duty cycle of the PWM signal. RECOMMENDED INPUT AND OUTPUT RATINGS True average output current operation is achieved with Input: 4.5~38VDC fast transient response by using cycle-by-cycle, Output: 1~10 LEDs in series/1.5A controlled on-time method. IS31LT3952 IS32LT3952 is available in an SOP-8-EP ABSOLUTE MAXIMUM RATINGS package with an exposed pad for enhanced thermal dissipation. It operates from 4.5V to 38V over the Input voltage 42VDC temperature range of -40C to +125C. Caution: Do not exceed the conditions listed above, otherwise the board will be damaged. FEATURES Wide input voltage supply from 4.5V to 38V PROCEDURE - Withstand 40V load dump The IS31LT3952 IS32LT3952 DEMO Board is fully 5% true average output current control assembled and tested. Follow the steps listed below to 1.5A maximum output over operating temperature verify board operation. range Caution: Do not turn on the power supply until all connections Cycle-by-cycle current limit are completed. Integrated high-side MOSFET switch 1) Connect the positive terminal of the power supply Dimming via direct logic input or power supply to the VCC of the board and the negative terminal voltage of the power supply to the GND of the board. Internal control loop compensation 2) Connect the negative of the LED panel (LED Under-voltage lockout (UVLO) and thermal arrays) to the LED- terminal. And connect the shutdown protection positive of the LED panel (LED arrays) to the 2 A low power shutdown LED+ terminal. Spread spectrum to optimize EMI 3) Select R register on the DEMO Board by SET Robust fault protection: JP1~JP4 to set output current, that I =0.2/R . OUT SET - Pin-to-GND short 4) Select EN/PWM pin to VCC by JP5 or connect to a PWM signal generator. Note: when connect to - Component open/short faults the PWM signal, the JP5 must be open to avoid - Adjacent pin-to-pin short PWM generator damage. - LED open/short 5) Turn on the power supply and the LED panels AEC-Q100 qualification (LED arrays) will be lighted up. - IS32LT3952 only Lumissil Microsystems www.lumissil.com 1 Rev. A, 06/25/2018 IS31LT3952 IS32LT3952 DEMO BOARD GUIDE ORDER INFORMATION Part No. Temperature Range Package IS31LT3952-GRLS4-EB -40C to +125C (Industrial) SOP-8-EP, Lead-free IS32LT3952-GRLA3-EB -40C to +125C (Automotive) For pricing, delivery, and ordering information, please contacts Lumissils analog marketing team at analog Lumissil.com or (408) 969-6600. DETAILED DESCRIPTION calculated by below Equation (2) and (3): OUTPUT CURRENT SETTING k R R V TON INT OUT t (2) ON The LED current is configured by an external sense V CC resistor, R , with a value determined as follows SET Equation (1): 1 f (3) SW k R R I V /R (1) TON INT LED FB SET Where k= 0.00458, with f in MHz, t in s, and R SW ON TON Where V = 0.2V (Typ.). FB and R (internal resistance, 20k ) in k. INT Note that R = 0.133 is the minimum allowed value SET Higher frequency gets smaller components size but of sense resistor to maintain switch current below the increases the switching losses and high-side MOSFET specified maximum value. gate driving current, and may not allow sufficiently high Table 1 R Resistance Versus Output Current SET or low duty cycle. Lower frequency gives better performance at larger components size. R ( ) Nominal Average Output Current (mA) SET INDUCTOR 0.4 500 Inductor value involves trade-offs in performance. 0.2 1000 Larger inductance reduces inductor current ripple that 0.133 1500 obtains smaller output current ripple, however it also brings in unwanted parasitic resistance that degrade The R should be a 1% resistor with enough power SET the performance. Smaller inductance has compact tolerance and good temperature characteristic to size and lower cost, but introduces higher ripple in the ensure accurate and stable output current. LED string. Use the following equations to estimate the FREQUENCY SELECTION approximate inductor value: During switching the IS31LT3952 IS32LT3952 (V V ) V CC LED LED operates in a consistent on-time mode. The on-time is L (4) f I V adjusted by an external resistor, R , which is TON SW L CC connected between the VCC and TON pins. Where V uses the minimum input voltage in volts, CC 2.2 V is the total forward voltage of LED string in volts, LED 2 f is the operation frequency in hertz. I is the SW L 1.8 current ripple in the inductor. Select an inductor with a 1.6 rating current over output average current and the 1.4 saturation current over the Over Current Protection (OCP) current threshold I . 1.2 SWLIM 1 Since IS31LT3952 IS32LT3952 is a Continuous 0.8 Conduction Mode (CCM) buck driver which means the 0.6 valley of the inductor current, I , should not drop to MIN zero all the time, the I must be smaller than 200% of 0.4 L the average output current. 0.2 0 I 0 100 200 300 400 500 600 700 800 900 1000 1100 L I I 0 (5) MIN LED RTON (k ) 2 Figure 2: Operating Frequency vs. R Resistance TON Besides, the peak current of the inductor, I , must be MAX smaller than I to prevent device from triggering SWLIM The approximate operating frequency can be Lumissil Microsystems www.lumissil.com 2 Rev. A, 06/25/2018 fSW (MHz)