LM3533 Datasheet, PDF (31/53 Pages) Texas Instruments – Complete Lighting Power Solution for Smartphone Handsets

LM3533
• Configure all LVLED strings for Anode connected to CPOUT (register 0x25 bits[6:2] = 1)
• Force the Charge Pump into 2X gain (Register 0x26 Bits[2:1] = 11). Ensure that CPOUT and CP are in the circuit and that
(V_CPOUT is > VF_LVLED + V_{HR_LV})
• Set Bank C full-scale Current to 20.2mA (Register 0x21 = 0x13)
• Set Bank C brightness to max (Register 0x42 = 0xFF)
• Set the startup ramp times to the fastest setting (Register 0x12 = 0x00)
• Assign LVLED1 - LVLED5 to Bank C (Register 0x11 = 0x00, Register 0x10 = 0x00)
• Enable Bank C (Register 0x27 Bit[2] = 1
• Wait 4ms
• Read back bits[6:2] of register 0xB0. 1 indicates an open and a 0 indicates normal operation (see Table 45)
• Disable all banks (Register 0x27 = 0x00)
SHORTED LED (LVLED)
The LM3533 features an LED short fault flag indicating when any active low-voltage LED is shorted (Anode to Cathode). A short
in an LVLED is determined when the LED voltage (V_CPOUT - V_HR) falls below 1V.
A procedure for determining a short in an LVLED string is detailed below:
• Apply Power
• Enable Short Fault (Register 0xB2, bit [1] = 1)
• Enable Feedback on the LVLED Current Sinks (Register 0x25 = 0xFF)
• Set Bank C full-scale current to 20.2mA (Register 0x21 = 0x13)
• Set Bank C brightness to max (Register 0x42 = 0xFF)
• Set the startup ramp times to the fastest setting (Register 0x12 = 0x00)
• Assign LVLED1 to LVLED5 to Bank C (Register 0x11 = 0x00, Register 0x10 = 0x00)
• Set Charge Pump to 1X gain (Register 0x26 = 0x40)
• Enable Bank C (Register 0x27 Bit[2] = 1
• Wait 4ms
• Read bits[6:2] from register 0xB1. A 1 indicates short, and a 0 indicates normal (see Table 46).
• Disable all banks (Register 0x27 = 0x00)
OVER-VOLTAGE PROTECTION (INDUCTIVE BOOST)
The over-voltage protection threshold (OVP) on the LM3533 has 4 different programmable options (16V, 24V, 32V, and 40V). The
OVP protects the device and associated circuitry from high voltages in the event the high-voltage LED string becomes open. During
normal operation, the LM3533’s inductive boost converter will boost the output up so as to maintain at least 400mV at the active,
high-voltage (COUT connected) current sink inputs. When a high-voltage LED string becomes open, the feedback mechanism is
broken, and the boost converter will over-boost the output. When the output voltage reaches the OVP threshold the boost converter
will stop switching, thus allowing the output node to discharge. When the output discharges to V_OVP – 1V the boost converter will
begin switching again. The OVP sense is at the OVP pin, so this pin must be connected directly to the inductive boost output
capacitor’s positive terminal.
For high-voltage current sinks that have the Anode Connect Register setting such that the high-voltage current sinks anodes are
not connected to COUT (feedback is disabled), the over-voltage sense mechanism is not in place to protect the input to the high-
voltage current sink. In this situation the application must ensure that the voltage at HVLED1 or HVLED2 doesn’t exceed 40V.
The default setting for OVP is set at 16V. For applications that require higher than 16V at the boost output, the OVP threshold must
be programmed to a higher level after powerup.
CURRENT LIMIT (INDUCTIVE BOOST)
The NMOS switch current limit for the LM3533’s inductive boost is set at 1A. When the current through the LM3533’s NFET switch
hits this over-current protection threshold (OCP), the device turns the NFET off and the inductor’s energy is discharged into the
output capacitor. Switching is then resumed at the next cycle. The current limit protection circuitry can operate continuously each
switching cycle. The result is that during high-output power conditions the device can continuously run in current limit. Under these
conditions the LM3533’s inductive boost converter stops regulating the headroom voltage across the high-voltage current sinks.
This results in a drop in the LED current.
CURRENT LIMIT (CHARGE PUMP)
The LM3533's charge pump's output current limit is set high enough so that the device will support 29.8mA (max full-scale current)
in all LVLED current sinks. This would typically be (29.5mA × 5 = 149mA. For 1X gain the output current limit is typically 350mA
(VIN = 3.6V). For 2X gain the current limit is typically 240mA (output referred), with a typical limit on the input current of 480mA.
The typical performance characteristic curves detail the charge pump current limit vs VIN at both 1X and 2X gain settings (see
Typical Performance Characteristics).
Copyright © 1999-2012, Texas Instruments Incorporated
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