MAX16816 Datasheet, PDF (18/33 Pages) Maxim Integrated Products – Programmable Switch-Mode LED Driver with Analog-Controlled PWM Dimming

Programmable Switch-Mode LED Driver
with Analog-Controlled PWM Dimming
where V_SENSE is the 200mV maximum differential volt-
age between SNS+ and SNS- and I_PEAK is the peak
inductor current at full load and minimum input voltage.
When the voltage drop across R_SENSE exceeds the
ILIM threshold, the MOSFET driver (DRV) terminates
the on-cycle and turns the switch off, reducing the cur-
rent through the inductor. The FET is turned back on at
the beginning of the next switching cycle.
When the voltage across R_SENSE exceeds the 300mV
(typ) HICCUP threshold, the HIC comparator terminates
the on-cycle of the device, turning the switching MOS-
FET off. Following a startup delay of 8ms (typ), the
MAX16816 reinitiates soft-start. The device will continue
to operate in HICCUP mode until the overcurrent condi-
tion is removed.
A programmable built-in leading-edge blanking circuit
of the current-sense signal prevents these comparators
from prematurely terminating the on-cycle of the exter-
nal switching MOSFET (Q_S). Select a blanking time
from 75ns to 150ns by configuring the Blanking Time
register in the EEPROM. In some cases, the maximum
blanking time may not be adequate and an additional
RC filter may be required to prevent spurious turn-off.
Load Current Sense
The load sense resistor, R_CS, monitors the current
through the LEDs. The internal floating current-sense
amplifier, CSA, measures the differential voltage across
R_CS, and generates a voltage proportional to the load
current through R_CS at CS. This voltage on CS is
referred to AGND. The closed-loop regulates the load
current to a value, I_LED, given by the following equation:
I_LED = V_SS / R_CS
where V_SS is the binning adjustment voltage. Set the value
of V_SS in the Binning Adjustment register in the EEPROM
between 100mV and 166mV. See the EEPROM and
Programming section for more information on adjusting
the binning voltage.
Slope Compensation
The amount of slope compensation required is largely
dependent on the down-slope of the inductor current
when the switching MOSFET, Q_S, is off. The inductor
down-slope depends on the input-to-output voltage dif-
ferential of the converter, the inductor value, and the
switching frequency. For stability, the compensation
slope should be equal to or greater than half of the
inductor current down-slope multiplied by the current-
sense resistance (R_SENSE).
See the EEPROM and Programming section for more infor-
mation on the ESLP register.
Internal Voltage-Error Amplifier (EAMP)
The MAX16816 includes a built-in voltage amplifier,
with three-state output, which can be used to close the
feedback loop. The buffered output current-sense sig-
nal appears at CS, which is connected to the inverting
input, FB, of the error amplifier through resistor R₁. The
noninverting input is connected to an internally trimmed
current reference.
The output of the error amplifier is controlled by the signal
applied to DIM. When DIM is high, the output of the ampli-
fier is connected to COMP. The amplifier output is open
when DIM is low. This enables the integrating capacitor to
hold the charge when the DIM signal has turned off the
gate drive. When DIM is high again, the voltage on the
compensation capacitors, C₁ and C₂, forces the converter
into steady state almost instantaneously.
PWM Dimming
PWM dimming is achieved by driving DIM with either a
PWM signal or a DC signal. The PWM signal is con-
nected internally to the error amplifier, the dimming
MOSFET gate driver, and the switching MOSFET gate
driver. When the DIM signal is high, the dimming MOSFET
and the switching MOSFET drivers are enabled and the
output of the voltage-error amplifier is connected to
the external compensation network. Also, the buffered
current-sense signal is connected to CS. Preventing
discharge of the compensation capacitor when the
DIM signal is low allows the control loop to return the
LED current to its original value almost instantaneously.
When the DIM signal goes low, the output of the error
amplifier is disconnected from the compensation net-
work and the compensation capacitors, C₁ and C₂,
voltage is preserved. Choose low-leakage capacitors
for C₁ and C₂. The drivers for the external dimming and
switching MOSFETs are disabled, and the converter
stops switching. The inductor energy is now transferred
to the output capacitors.
When the DIM signal goes high and the gate drivers
are enabled, the additional voltage on the output
capacitor may cause a current spike on the LED string.
A larger output capacitor will result in a smaller current
spike. If the overcurrent spike exceeds 30% of the pro-
grammed LED current, the dimming is turned off and
the MAX16816 reinitiates soft-start.
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