ADD5211 Data Sheet
Rev. 0 | Page 10 of 20
THEORY OF OPERATION
The ADD5211 uses a PWM boost controller to generate the
minimum output voltage required to drive the LED string at the
programmed LED current. The current mode control architecture
allows a fast transient response while maintaining a stable output
voltage. The boost converter provides power to the LED strings,
and the four current sinks control the LED current with dynamic
headroom control to improve efficiency.
CURRENT MODE, STEP-UP SWITCHING
CONTROLLER
The ADD5211 is a current mode, PWM boost controller
that operates at a fixed switching frequency from 200 kHz to
1.2 MHz. The switching frequency is set by an external resistor
connected from the FREQ pin to AGND. The minimum head-
room voltage—which is monitored at the FB1, FB2, FB3, and
FB4 pins—is compared with the internal reference voltage by
the internal transconductance error amplifier to create an error
current at COMP. A resistor and capacitor connected from the
COMP pin to AGND convert the error current to an error
voltage.
At the beginning of the switching cycle, the MOSFET is turned
on and the inductor current ramps up. The MOSFET current is
measured and converted to a voltage using the current sense
resistor (RCS) and is added to the stabilizing slope compensation
ramp from the ramp resistor (RRAMP). The resulting voltage sum
passes through the current sense amplifier to generate the
current sense voltage. Under light loads, the converter can also
operate in discontinuous mode with pulse skip modulation to
maintain output voltage regulation.
The current mode regulation system of the ADD5211 allows
fast transient responses while maintaining a stable output
voltage. By selecting the proper resistor-capacitor network from
COMP to AGND, the regulator response can be optimized for a
wide range of input voltages, output voltages, and load currents.
Input Voltage
The ADD5211 can be powered directly from the VIN pin,
which accepts a voltage from 4.5 V to 40 V. The voltage on the
VIN pin must exceed VUVLOR_VIN (4.0 V typical) for startup. The
ADD5211 has two linear regulators: a 3.3 V linear regulator
(VDD), which supplies power to the internal control circuitry,
and a 5.1 V linear regulator (VDR), which supplies power to the
internal GATE_P and GATE_N drivers.
UVLO Pin
The UVLO pin is used to control the VIN voltage at which
the ADD5211 starts up. This function is accomplished using
a resistor divider between the input voltage and the UVLO pin,
as shown in Figure 16.
VIN
10555-016
R
UVLO1
R
UVLO2
UVLO_REF
UVLO
PIN
Figure 16. Undervoltage Lockout Circuit
The startup voltage, as determined by the resistor divider to the
UVLO pin, can be calculated using the following equation:
VIN(START) = (1.19 V/RUVLO2) × (RUVLO1 + RUVLO2)
To start the device at the lowest possible VIN level, select an
RUVLO1 value of 100 kΩ (or greater) and do not connect RUVLO2.
If UVLO is controlled from a separate voltage source, make sure
that a 100 kΩ (or greater) resistor is in series between the voltage
source and the UVLO pin.
Enable and Disable
To enable the ADD5211, the voltage at the EN pin must be
higher than 2.2 V. To disable the ADD5211, the voltage at the
EN pin must be lower than 0.8 V. An internal 500 kΩ resistor is
connected between EN and AGND.
Internal 3.3 V Regulator (VDD)
The ADD5211 contains a 3.3 V linear regulator (VDD) that is
used to bias the internal control circuitry. The VDD regulator
requires a 1 µF bypass capacitor. Place this bypass capacitor
between VDD and AGND, as close as possible to the VDD pin.
Internal 5.1 V Regulator (VDR)
The ADD5211 contains a 5.1 V linear regulator (VDR) that is
used to supply the MOSFET gate driver. The VDR regulator
requires a 1 µF bypass capacitor. Place this bypass capacitor
between VDR and AGND, as close as possible to the VDR pin.