MAX16821A_09 Datasheet, PDF (10/24 Pages) Maxim Integrated Products – High-Power Synchronous HBLED Drivers with Rapid Current Pulsing

High-Power Synchronous HBLED
Drivers with Rapid Current Pulsing
Synchronization
The MAX16821A/MAX16821B/MAX16821C synchronize
to an external clock connected to RT/SYNC. The appli-
cation of an external clock at RT/SYNC disables the
internal oscillator. Once the MAX16821A/MAX16821B/
MAX16821C are synchronized to an external clock, the
external clock cannot be removed if reliable operation
is to be maintained.
Control Loop
The MAX16821A/MAX16821B/MAX16821C use an
average current-mode control scheme to regulate the
output current (Figure 2). The main control loop con-
sists of an inner current regulation loop for controlling
the inductor current and an outer current regulation
loop for regulating the LED current. The inner current
regulation loop absorbs the double pole of the inductor
and output capacitor combination reducing the order of
the outer current regulation loop to that of a single-pole
system. The inner current regulation loop consists of a
current-sense resistor (R_S), a current-sense amplifier
(CSA), a current-error amplifier (CEA), an oscillator pro-
viding the carrier ramp, and a PWM comparator
(CPWM) (Figure 2). The MAX16821A/MAX16821B/
MAX16821C outer LED-current control loop consists of
a differential amplifier (DIFF), a reference voltage, and
a voltage-error amplifier (VEA).
Inductor Current-Sense Amplifier
The differential current-sense amplifier (CSA) provides a
34.5V/V DC gain. The typical input offset voltage of the
current-sense amplifier is 0.1mV with a 0 to 5.5V common-
mode voltage range (V_IN = 7V to 28V). The current-sense
amplifier senses the voltage across R_S. The maximum
common-mode voltage is 3.2V when V_IN = 5V.
Inductor Peak-Current Comparator
The peak-current comparator provides a path for fast
cycle-by-cycle current limit during extreme fault condi-
tions, such as an inductor malfunction (Figure 3). Note
the average current-limit threshold of 27.5mV still limits
the output current during short-circuit conditions. To
prevent inductor saturation, select an inductor with a
saturation current specification greater than the aver-
age current limit. The 60mV threshold for triggering the
peak-current limit is twice the full-scale average cur-
rent-limit voltage threshold. The peak-current compara-
tor has only a 260ns delay.
R_IN
R_F
C_F
R_CF ^CCP
C_CZ
DIFF
EAN
EAOUT CSN CSP
CLP
SENSE+
DIFF
SENSE-
VEA
V_REF
CA
CEA
V_IN
CPWM
DRIVER
MODE = SGND
L
LED
STRING
C_OUT
R_S
R_LS
Figure 2. MAX16821A/MAX16821B/MAX16821C Control Loop
10 ______________________________________________________________________________________