Rainbow Electronics MAX6948B - Manual (Page 13)

High-Efficiency PWM LED Driver with Boost
Converter and Five Constant-Current GPIO Ports
13
MAX6948B
Boost-Converter Shutdown/Standby Modes
The boost converter shuts down when D0 of the con-
figuration register (0x10) = 0, or when D0 of the boost-
converter status register (0x22) = 1. If both the boost
PWM output registers’ (0x20, 0x21) values are zero, the
boost converter remains in a low-current state (standby).
Undervoltage Lockout (UVLO)
Undervoltage lockout (UVLO) disables the boost con-
verter when V+ is below 2.4V (max). This resets bit D0
of the configuration register and puts the part into shut-
down mode (0x10).
Quick Start
The MAX6948B quick starts by charging C COMP with a
current source. During this time, the internal MOSFET
is switching at the minimum duty cycle. Once V
COMP
rises above 0.2V, the duty cycle increases until the
output voltage reaches the desired regulation level. In
shutdown mode, COMP is pulled to GND with a 20kI
internal resistor.
Overvoltage Protection
If the voltage on the output terminal rises above 28.5V
(min), the converter is put into standby mode. This pro-
tects the converter from excessive voltage in the event
of an open-circuit condition. To detect if the boost con-
verter has exceeded the overvoltage limit, read bit D1
of the boost-converter status register (0x22). Once the
output voltage has dropped 4V below the overvoltage
threshold, the read-only bit (D1) goes to zero. The boost
converter leaves standby mode and normal operation
resumes. Reading the register causes the bit to reset. If
the fault is still active, the bit will be set again.
Thermal Shutdown
Thermal shutdown limits total power dissipation in the
MAX6948B. When the junction temperature exceeds
151NC (typ), the boost converter and ports P0–P4 turn
off, allowing the part to cool. The thermal shutdown bit
(D2) of the boost configuration and status register (0x22)
is set high. Bit D0 of the boost-converter status register
(0x22) = 1, bit D0 of the configuration register (0x10)
= 0 (reset), and the device is in shutdown mode. The
MAX6948B turns on and begins to quick-start after the
junction temperature cools by 10NC. Reading this regis-
ter causes the bit to reset. If the fault is still active, the bit
will be set again.
Current Limit
The MAX6948B current-limit function monitors the induc-
tor current when the internal switch on the LX node is
on. The device compares the inductor current to a fixed
threshold. When the current exceeds the threshold, bit
D3 of the boost-converter status register asserts and
the switch shuts off for that cycle. Reading this register
causes the bit to reset. If the fault is still active, the bit
will be set again.
Boost-Converter Current Settings
The boost current, through the serial output LEDs, can
be set to half or full scale by setting the FB pin voltage.
The FB voltage is set through bit D7 of the configuration
register (0x10) (Table 2). The FB voltage settings are
100mV or 200mV for half- or full-current mode operation,
respectively.