7.3.1.1 Current Sink Enable

Each current sink in the device has a separate enable input. This allows for a 1-string or 2-string application. The default is with two strings enabled. Once the correct LED string configuration is programmed, the device can be enabled by writing the chip enable bit high (register 0x10 bit[0]), and then either enabling PWM and driving PWM high, or writing a non-zero code to the brightness registers.

Table 1. LM36922H Operating Modes

7.3.3.1 Linear Mapping

For linear mapped mode the LED current increases proportionally to the 11-bit brightness code and follows the relationship:

Equation 1.

This is valid from codes 1 to 2047. Code 0 programs 0 current. Code is an 11-bit code that can be the I²C brightness code, the digitized PWM duty cycle, or the product of the two.

7.3.3.2 Exponential Mapping

In exponential mapped mode the LED current follows the relationship:

Equation 2.

This results in an LED current step size of approximately 0.304% per code. This is valid for codes from 1 to 2047. Code 0 programs 0 current. Code is an 11-bit code that can be the I²C brightness code, the digitized PWM duty cycle, or the product of the two. Figure 17 details the LED current exponential response.

The 11-bit (0.304%) per code step is small enough such that the transition from one code to the next in terms of LED brightness is not distinguishable to the eye. This therefore gives a perfectly smooth brightness increase between adjacent codes.

Figure 17. LED Current vs Brightness Code (Exponential Mapping)

7.3.4.1 PWM Sample Frequency

There are four selectable sample rates for the PWM input. The choice of sample rate depends on three factors:

1. Required PWM Resolution (input duty cycle to brightness code, with 11 bits max)
2. PWM Input Frequency
3. Efficiency

7.3.4.1.1 PWM Resolution and Input Frequency Range

The PWM input frequency range is 50 Hz to 50 kHz. To achieve the full 11-bit maximum resolution of PWM duty cycle to the LED brightness code (BRT), the input PWM duty cycle must be ≥ 11 bits, and the PWM sample period (1/ƒ_SAMPLE) must be smaller than the minimum PWM input pulse width. Figure 18 shows the possible brightness code resolutions based on the input PWM frequency. The minimum PWM frequency for each PWM sample rate is described in PWM Timeout.

Figure 18. PWM Sample Rate, Resolution, and PWM Input Frequency

7.3.4.2 PWM Hysteresis

To prevent jitter at the input PWM signal from feeding through the PWM path and causing oscillations in the LED current, the LM36922H offers seven selectable hysteresis settings. The hysteresis works by forcing a specific number of 11-bit LSB code transitions to occur in the input duty cycle before the LED current changes. Table 4 describes the hysteresis. The hysteresis only applies during the change in direction of brightness currents. Once the change in direction has taken place, the PWM input must over come the required LSB(s) of the hysteresis setting before the brightness change takes effect. Once the initial hysteresis has been overcome and the direction in brightness change remains the same, the PWM to current response changes with no hysteresis.

Figure 19. PWM Hysteresis Example

7.3.4.3 PWM Step Response

The LED current response due to a step change in the PWM input is approximately 2 ms to go from minimum LED current to maximum LED current.

7.3.4.4 PWM Timeout

The LM36922H PWM timeout feature turns off the boost output when the PWM is enabled and there is no PWM pulse detected. The timeout duration changes based on the PWM Sample Rate selected which results in a minimum supported PWM input frequency. The sample rate, timeout, and minimum supported PWM frequency are summarized in Table 5.

7.4.1.1 I²C Only (Brightness Mode 00)

In brightness control mode 00 the I²C Brightness registers are in control of the LED current, and the PWM input is disabled. The brightness data (BRT) is the concatenation of the two brightness registers (3 LSBs) and (8 MSBs) (registers 0x18 and 0x19, respectively). The LED current only changes when the MSBs are written, meaning that to do a full 11-bit current change via I²C, first the 3 LSBs are written and then the 8 MSBs are written. In this mode the ramper only controls the time from one I²C brightness set-point to the next (see Figure 21).

Figure 21. Brightness Control 00 (I²C Only)

1. At time t0 the I²C Brightness Code is changed from 0x444 (1092d) to 0x7FF (2047d)
2. Ramp Rate programmed to 1ms/step
3. Mapping Mode set to Linear
4. ILED_t0 = 1092 × 12.213 µA = 13.337 mA
5. ILED_t1 = 2047 × 12.213 µA = 25 mA
6. t_RAMP = (t1 – t0) = 1ms/step × (2047 – 1092 – 1) = 954 ms

Figure 22. I²C Brightness Mode 00 Example (Ramp Between I²C Code Changes)

7.4.1.2 PWM Only (Brightness Mode 01)

In brightness mode 01, only the PWM input sets the brightness. The I²C code is ignored. The LM36922H samples the PWM input and determines the duty cycle; this measured duty cycle is translated into an 11-bit digital code. The resultant code is then applied to the internal ramper (see Figure 23).

Figure 23. Brightness Control 01 (PWM Only)

1. At time t0 the PWM duty cycle changed from 25% to 100%
2. Ramp Rate programmed to 1 ms/step
3. Mapping Mode set to Linear
4. ILED_t0 = 25 mA × 0.25 = 6.25 mA
5. ILED_t1 = 25 mA × 1 = 25 mA
6. t_RAMP = (t1 – t0) = 1 ms/step × (2047 × 1 – 2047 × 0.25 – 1) = 1534 ms

Figure 24. Brightness Control Mode 01 Example (Ramp Between Duty Cycle Changes)

7.4.1.3 I²C + PWM Brightness Control (Multiply Then Ramp) Brightness Mode 10

In brightness control mode 10 the I²C Brightness register and the PWM input are both in control of the LED current. In this case the I²C brightness code is multiplied with the PWM duty cycle to produce an 11-bit code which is then sent to the ramper. In this mode ramping is achieved between I²C and PWM currents (see Figure 25).

Figure 25. Brightness Control 10 (I²C + PWM)

1. At time t0 the I²C Brightness code changed from 0x444 (1092d) to 0x7FF (2047d)
2. At time t0 the PWM duty cycle changed from 50% to 75%
3. Ramp Rate programmed to 1ms/step
4. Mapping Mode set to Linear
5. ILED_t0 = 1092 × 12.213 µA × 0.5 = 6.668 mA
6. ILED_t1 = 2047 × 12.213 µA × 0.75 = 18.75 mA
7. t_RAMP = (t1 – t0) = 1 ms/step × (2047 × 0.75 – 1092 × 0.5 – 1) = 988 ms

Figure 26. Brightness Control Mode 10 Example (Multiply Duty Cycle then Ramp)

7.4.1.4 I²C + PWM Brightness Control (Ramp Then Multiply) Brightness Mode 11

In brightness control mode 11 both the I²C brightness code and the PWM duty cycle control the LED current. In this case the ramper only changes the time from one I²C brightness code to the next. The PWM duty cycle is multiplied with the I²C brightness code at the output of the ramper (see Figure 27).

Figure 27. Brightness Control 11 (I²C + PWM)

1. At time t0 the I²C Brightness code changed from 0x444 (1092d) to 0x7FF (2047d)
2. At time t0 the PWM duty cycle changed from 50% to 75%
3. Ramp Rate programmed to 1 ms/step
4. Mapping Mode set to Linear
5. ILED_t0– = 1092 × 12.213 µA × 0.5 = 6.668 mA
6. ILED_t0+ = 1092 × 12.213 µA × 0.75 = 10.002 mA
7. t_RAMP = (t1 – t0) = 1 ms/step × (2047 – 1092 – 1) = 954 ms

Figure 28. Brightness Control Mode 11 Example (Ramp Current Then Multiply Duty Cycle)

7.4.2.1 Minimum Inductor Select

The LM36922H can use inductors in the range of 4.7 µH to 10 µH. In order to optimize the converter response to changes in V_IN and load, the Min Inductor Select bit (register 0x13 bit[4]) should be selected depending on which value of inductance is chosen. For 10-µH inductors this bit should be set to 1. For less than 10 µH, this bit should be set to 0.

Table 6. Auto-Switching Frequency Operation

Table 7. Auto Frequency 250-kHz Threshold Settings

7.4.5.1 Overvoltage Protection (OVP)

The LM36922H provides four OVP thresholds (17 V, 24 V, 32 V, and 38 V). The OVP circuitry monitors the boost output voltage (V_OUT) and protects OUT and SW from exceeding safe operating voltages in case of open load conditions or in the event the LED string voltage requires more voltage than the programmed OVP setting. The OVP thresholds are programmed in register 13 bits[3:2]. The operation of OVP differentiates between two overvoltage conditions (see Case 1 OVP Fault Only (OVP Threshold Hit and All Enabled Current Sink Inputs > 40 mV), Case 1 OVP Fault Only (OVP Threshold Hit and All Enabled Current Sink Inputs > 40 mV), and Case 2b OVP Fault and Open LED String Fault (OVP Threshold Duration and Any Enabled Current Sink Input ≤ 40 mV)).

7.4.5.2 Voltage Limitations on LED1, LED2

The inputs to current sinks LED1, LED2 are rated for 30 V (absolute maximum voltage). This is lower than the boost output capability as set by the OVP threshold (maximum specification) of 39 V. To ensure that the current sink inputs remain below their absolute maximum rating, the LED configuration between LED1 or LED2 must not have a voltage difference between strings so that VLED1/2 have a voltage greater than 30 V.

7.4.5.3 LED String Short Fault

The LM36922H can detect an LED string short fault. This happens when the voltage between V_IN and any enabled current sink input has dropped below (1.5 V). This test can only be performed on one LED string at a time. Performing this test with more than one LED string enabled can result in a faulty reading. The procedure for detecting a short in a LED string is:

• Apply power the LM36922H .
• Enable only LED1 string (Register 0x10 = 0x03).
• Enable short fault (Register 0x1E = 0x01.
• Set maximum brightness (Register 0x18 = 0x07 and Register 0x19 = 0xFF).
• Set the brightness control (Register 0x11 = 0x00).
• Wait 4 msec.
• Read LED short fault (Register 0x1F).
• If bit[3] = 1, then a LED short fault condition has been detected.
• Set chip enable and LED string enable low (Register 0x10 = 0x00).
• Repeat the procedure for the other LED strings.

7.4.5.4 Overcurrent Protection (OCP)

The LM36922H has four selectable OCP thresholds (750 mA, 1000 mA, 1250 mA, and 1500 mA). These are programmable in register 0x13 bits[1:0]. The OCP threshold is a cycle-by-cycle current limit and is detected in the internal low-side NFET. Once the threshold is hit the NFET turns off for the remainder of the switching period.

7.4.5.5 Device Overtemperature

Thermal shutdown (TSD) is triggered when the device die temperature reaches 135˚C. When this happens the boost stops switching, and the TSD Flag (register 0x1F bit[2]) is set. The boost automatically starts up again when the die temperature cools down to 120°C.

7.5.1.1 Start and Stop Conditions

The LM36922H is configured via an I²C interface. START (S) and STOP (P) conditions classify the beginning and the end of the I²C session Figure 29. A START condition is defined as SDA transitioning from HIGH to LOW while SCL is HIGH. A STOP condition is defined as SDA transitioning from LOW to HIGH while SCL is HIGH. The I²C master always generates the START and STOP conditions. The I²C bus is considered busy after a START condition and free after a STOP condition. During the data transmission the I²C master can generate repeated START conditions. A START and a repeated START conditions are equivalent function-wise. The data on SDA must be stable during the HIGH period of the clock signal (SCL). In other words, the state of SDA can only be changed when SCL is LOW.

Figure 29. I²C Start and Stop Conditions

7.5.1.2 I²C Address

After the START condition the I²C master sends the 7-bit chip address followed by an eighth bit read or write (R/W). R/W = 0 indicates a WRITE, and R/W = 1 indicates a READ. The second byte following the chip address selects the register address to which the data is written. The third byte contains the data for the selected register.

7.5.1.3 Transferring Data

Every byte on the SDA line must be eight bits long with the most significant bit (MSB) transferred first. Each byte of data must be followed by an acknowledge bit (ACK). The acknowledge related clock pulse, (9th clock pulse), is generated by the master. The master then releases SDA (HIGH) during the 9th clock pulse. The LM36922H pulls down SDA during the 9th clock pulse, signifying an acknowledge. An acknowledge is generated after each byte has been received.

7.5.1.4 Register Programming

For glitch free operation, the following bits and/or registers should only be programmed while the LED Enable bits are 0 (Register 0x10, Bit [2:1] = 0) and Device Enable bit is 1 (Register 0x10, Bit[0] = 1) :

1. Register 0x11 Bit[7] (Mapping Mode)
2. Register 0x11 Bits[6:5] (Brightness Mode)
3. Register 0x11 Bit[4] (Ramp Enable)
4. Register 0x11 Bit[3:1] (Ramp Rate)
5. Register 0x12 Bits[7:6] (PWM Sample Rate)
6. Register 0x12 Bits[5] (PWM Polarity)
7. Register 0x12 Bit[3:2] (PWM Hysteresis)
8. Register 0x12 Bit[3:2] (PWM Pulse Filter)
9. Register 0x15 (auto frequency high threshold)
10. Register 0x16 (auto frequency low threshold)