MAX16826ATJ+T Maxim Integrated Products, MAX16826ATJ+T Datasheet - Page 14

• Manufacturer Part Number: MAX16826ATJ+T
• Description: LED Drivers Programmable Four-St ring HB LED Driver w
• Manufacturer: Maxim Integrated Products

Specifications of MAX16826ATJ+T

• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

Programmable, Four-String HB LED Driver with
Output-Voltage Optimization and Fault Detection
The MAX16826 oscillator frequency is programmable
using an external capacitor (C33 in the Typical
Application Circuit ) and a resistor (R19) at RTCT. R19 is
connected from RTCT to V
from RTCT to GND. C33 charges through RT until V
reaches 2.85V. CT then discharges through an 8.4mA
internal current sink until V
then allowed to charge through R19 again. The period
of the oscillator is the sum of the charge and discharge
times of C3. Calculate these times as follows:
The charge time is:
The discharge time is:
where t
C33 is in farads (F).
The oscillator frequency is then:
The charge time (t
sets the maximum duty cycle of the switching regulator.
Therefore, the charge time (t
desired maximum duty cycle. Typically, the duty cycle
should be limited to 95%. The oscillator frequency is
programmable from 100kHz to 1MHz. The MAX16826
can be synchronized to an external oscillator through
SYNC/EN.
The MAX16826 uses an internal ramp generator for
slope compensation to stabilize the current loop when
the duty cycle exceeds 50%. A slope compensation
resistor (R17 in the Typical Application Circuit ) is con-
nected between RSC and the switching current-sense
resistor at the source of the external switching FET.
When the voltage at DL transitions from low to high, a
ramped current with a slope of 26µA/µs is generated
and flows through the slope compensation resistor. It is
effectively summed with the current-sense signal. When
the voltage at DL is low, the current ramp is reset to 0.
Calculate R17 as follows:
where V
V
14
INMIN
t
D
=
______________________________________________________________________________________
R
C
is the minimum operating input voltage.
19
OUT
and t
×
C
R
is the switching regulator output and
33
D
17
t
is in seconds, R19 is in ohms (Ω), and
C
×
=
C
= 0.55 x R19 x C33
ln
f
(
OSC
) in relation to the period (t
V
OUT
(
(
R
19
=
34 28
Slope Compensation (RSC)
−
t
RTCT
−
V
C
CC
.
INMIN
281 86
1
+
C
t
) is constrained by the
×
and C33 is connected
D
drops to 1.2V. C33 is
L
.
1
)
×
)
R
(
12
R
19
−
Oscillator
487 4 4 5
C
.
+ t
RTCT
)
)
D
)
The MAX16826 includes a primary cycle-by-cycle, cur-
rent-limit comparator and a secondary gross current-
limit comparator to terminate the on-time or switch
cycle during an overload or fault condition. The current-
sense resistor (R12 in the Typical Application Circuit )
connected between the source of the switching FET
and GND and the internal threshold, set the current
limit. The current-sense input (CS) has a voltage trip
level (V
culate R39:
where I
switching FET. When the voltage across R12 exceeds
the current-limit comparator threshold, the FET driver
(DL) turns the switch off within 80ns. In some cases, a
small RC filter may be required to filter out the leading-
edge spike on the sensed waveform. Set the time con-
stant of the RC filter at approximately 100ns and adjust
as needed.
If, for any reason, the voltage at CS exceeds the 270mV
trip level of the gross current limit as set by a second
comparator, then the switching cycle is immediately
terminated and the soft-start capacitor is discharged.
This allows a new soft-start cycle and prevents inductor
current buildup.
Soft-start is achieved by charging the external soft-start
capacitor (C30 in the Typical Application Circuit ) at
startup. An internal fixed 6µA current source charges
the soft-start capacitor until V
achieve the required soft-start timing for the switching
regulator output voltage to reach regulation, the value
of the soft-start capacitor at CSS is calculated as:
where t
regulator output regulation and V
lation voltage. When the IC is disabled, the soft-start
capacitor is discharged to GND.
The SYNC/EN input provides both external clock syn-
chronization (if desired) and enable control. When
SYNC/EN is held low, all circuits are disabled and the
IC enters low-current shutdown mode. When SYNC/EN
is high, the IC is enabled and the switching regulator
clock uses the RTCT network to set the operating fre-
quency. See the Oscillator section for details. The
SYNC/EN can also be used for frequency synchroniza-
tion by connecting it to an external clock signal from
100kHz to 1MHz. The switching cycle initiates on the
CS
SS
PK
) of 200mV. Use the following equation to cal-
is the required time to achieve the switching
is the peak current that flows through the
C30 = 6µA x t
Synchronization and Enable Input
R12 = V
CS
SS
/I
PK
CSS
/V
REF
REF
Current Limit (CS)
is the set FB regu-
reaches V
Soft-Start (CSS)
CC
. To