TI LM51772 buck-boost controller

The LM51772 driver (drivers/power/supply/lm51772.c) is a lower half of the SMPS driver for the Texas Instruments LM51772 buck-boost controller, controlled over I2C. It is enabled with CONFIG_SMPS_LM51772, which needs CONFIG_DRIVERS_SMPS and CONFIG_I2C. The register map and the bit fields, with the names of the datasheet, are in include/nuttx/power/lm51772.h.

The converter regulates a constant output voltage (SMPS_OPMODE_CV, the only mode) with an output current limit. The driver reports the state and the faults, but it has no output feedback.

Board interface

The board registers the device with:

int lm51772_register(const char *path, const struct lm51772_config_s *config);

and describes the hardware in struct lm51772_config_s:

Field	Description
`i2c`	I2C bus
`frequency`	I2C frequency
`addr`	7-bit address, 0x6a if 0
`r_isns_mohm`	Output sense resistor, in milliohms, which sets the scale of the current limit
`v_out_min`,	Absolute maximum ratings of the board, 0 for none.
`v_out_max`,	See below.
`i_out_max`,
`p_out_max`
`enable`, `arg`	Callback that drives the EN/UVLO pin. It is called with interrupts disabled when switching off, so it must not sleep.
`init`, `ninit`	Registers written at start, in order. See below.

Start and stop

The controller is enabled and configured by PWRIOC_START, not when the device is opened, because the upper half calls the lower half setup with interrupts disabled and the configuration needs I2C:

The enable callback is called and the driver waits 15 ms for the supplies of the controller.
The driver reads STATUS_BYTE to check that the controller answers. Otherwise the enable callback is called again to disable it and the error is returned by PWRIOC_START.
The registers of init are written in order, and the step of the output voltage is read back from MFR_SPECIFIC_D8 (SEL_FB_DIV20).
The output voltage target and the current limit are written, and the latched faults are cleared.
CONV_EN and CONV_EN2 are set, and FORCE_DISCH is cleared.

PWRIOC_STOP clears CONV_EN and CONV_EN2 and sets FORCE_DISCH, which discharges the output. When the device is closed for the last time the enable callback disables the controller and the converter stops, so a program that starts it has to keep the device open while it runs.

Register list

init lists the registers that the board needs to change from the values loaded by the controller. MFR_SPECIFIC_D0 is written with CONV_EN cleared, since the power stage starts only with PWRIOC_START. If the output voltage step is not the default one (MFR_SPECIFIC_D8), the entry must be in the list. An ILIM_THRESHOLD entry sets the current limit at start, when the parameters keep it, and LM51772_ILIM_THRESHOLD(ma, mohm) computes its value.

Absolute maximum ratings

The limits (PWRIOC_SET_LIMITS) and the parameters (PWRIOC_SET_PARAMS) set by the application are refused with -EINVAL when they exceed the ratings of the board: an output voltage outside of v_out_min to v_out_max, a current above i_out_max, or a power v_out * i_out above p_out_max. A current or power limit left at 0 becomes the rating. When the parameters keep the current limit, the current of the init ILIM_THRESHOLD entry is used for the power check.

The ILIM_THRESHOLD register saturates at 70 mV across the sense resistor (0x8c), and a current limit that needs a higher value is refused with -EINVAL.

Faults

The faults come from STATUS_BYTE:

`STATUS_BYTE`	`smps_fault_e`
`VOUT` (VOUT_OV)	`SMPS_FAULT_OVERVOLTAGE`
`IOUT` (IOUT_OC)	`SMPS_FAULT_OVERCURRENT`
`TEMPERATURE`	`SMPS_FAULT_OVERTEMP`
`INPUT` (VIN_UV),	`SMPS_FAULT_OTHER`
`CML`, `OTHER`

OFF and BUSY are not faults. A fault state is reported by PWRIOC_GET_STATE as SMPS_STATE_FAULT. The faults are latched by the controller: after a fault disappears, for example when the input voltage returns above the VDET rising threshold and the converter restarts, they stay set until PWRIOC_CLEAN_FAULT.

Hardware notes

These points depend on how the board connects the controller:

The EN/UVLO pin only enables the power stage. If the nRST pin is tied to the input, the logic and the I2C interface are always on while the input is present, and the controller keeps its registers across an enable callback with false and a reset of the processor. Only removing the input returns the registers to their defaults.
The MODE pin selects forced PWM (high) or the power save mode (low). In forced PWM the current can reverse and return to the input, so a source that does not sink current needs the input overvoltage protection (EN_IVP in MFR_SPECIFIC_D3 and the threshold in IVP_VOLTAGE).
With no external UVLO divider on the EN/UVLO pin, the VDET thresholds (VDET_FALL in MFR_SPECIFIC_D3 and VDET_RISE in MFR_SPECIFIC_D4) set the input undervoltage lockout.
EN_DTRK_STARTOVER (MFR_SPECIFIC_D1) starts the converter without waiting for the PWM signal of the DTRK pin. It is set at reset, and a board with the pin tied low should keep it set: on such a board the converter did not start again after a stop with the bit cleared.
SEL_ISET_PIN (MFR_SPECIFIC_D9, set at reset) disables the internal current limit DAC, and the limit would come from a resistor on the ILIMCOMP/ISET pin. A board without that resistor needs it cleared, or ILIM_THRESHOLD has no effect.
The nFLT/nINT pin is an open drain output that is low while a fault condition is active, OFF included, and high impedance when the condition goes away, even if the bit stays latched in STATUS_BYTE.

Example

A board that describes a 8 milliohm sense resistor, a 9 to 24 V output, 8 A and 120 W, and writes one register:

static const struct lm51772_reg_s g_init[] =
{
  { LM51772_REG_MFR_SPECIFIC_D1,
    LM51772_D1_EN_THER_WARN | LM51772_D1_EN_DTRK_STARTOVER },
  { LM51772_REG_ILIM_THRESHOLD, LM51772_ILIM_THRESHOLD(5000, 8) }
};

static void board_smps_enable(void *arg, bool enable)
{
  /* Drive the EN/UVLO pin */
}

int board_smps_init(struct i2c_master_s *i2c)
{
  struct lm51772_config_s config;

  config.i2c         = i2c;
  config.frequency   = 400000;
  config.addr        = 0;
  config.r_isns_mohm = 8;
  config.v_out_min   = 9.0f;
  config.v_out_max   = 24.0f;
  config.i_out_max   = 8.0f;
  config.p_out_max   = 120.0f;
  config.enable      = board_smps_enable;
  config.arg         = NULL;
  config.init        = g_init;
  config.ninit       = nitems(g_init);

  return lm51772_register("/dev/smps0", &config);
}