MAX16826_11 Datasheet, PDF (12/26 Pages) Maxim Integrated Products – Programmable, Four-String HB LED Driver with Output-Voltage Optimization and Fault Detection

Programmable, Four-String HB LED Driver with
Output-Voltage Optimization and Fault Detection
Shunt Regulator
The MAX16826 has an internal 26V (typ) shunt regula-
tor to provide the primary protection against an auto-
motive load dump. When the input voltage is below
26V, the shunt voltage at IN tracks the input voltage.
When the input voltage exceeds 26V, the shunt regula-
tor turns on to sink current, and the voltage at IN is
clamped to 26V. During a load dump, the input voltage
can reach 40V, and the shunt regulator through the
resistor connected to IN is forced to sink large amounts
of current for up to 400ms to limit the voltage that
appears at IN to the shunt regulation voltage. The sink-
ing current of the shunt regulator is limited by the value
of resistor (R1 in Figure 1) in series with IN. There are
two criteria that determine the value of R1: the maxi-
mum acceptable shunt current during load dump, and
the voltage drop on R1 under normal operating condi-
tions with low battery voltage. For example, with typical
20mA input current in normal operation, 250mA load
dump current limit, 40V maximum load dump voltage,
the R1 value is:
R1 =
V_INMIN − V_INREG
I_Q
=
7.5 − 5.5
20 × 10^−3
= 100Ω
where V_INMIN is the minimum operating voltage and
V_INREG is the minimum acceptable voltage at IN.
Use the following equation to verify that the current
through R1 is less than 250mA under a load-dump con-
dition:
I_LD
=
V_LD − 26V
R1
=
40 − 26
100
= 140mA
For stable operation, the shunt regulator requires a min-
imum 10μF of ceramic capacitance from IN to GND.
V_CC Regulator
The 5.25V V_CC regulator provides bias for the internal
circuitry including the bandgap reference and gate dri-
vers. Externally bypass V_CC with a minimum 4.7μF
ceramic capacitor. V_CC has the ability to supply up to
50mA of current, but external loads should be mini-
mized so as not to take away drive capability for inter-
nal circuitry. If IN is powered by a voltage less than
5.5V, connect V_CC directly to IN.
Switch-Mode Controller
The MAX16826 consists of a current-mode controller
that is capable of operating in the 100kHz to 1MHz fre-
quency range (Figure 2). Current-mode control pro-
vides fast response and simplifies loop compensation.
The error amplifier compares the voltage at FB to 1.25V
and varies the COMP output accordingly to regulate.
The PWM comparator compares the voltage at COMP
with the voltage at RSC to determine the switching duty
cycle. The primary cycle-by-cycle current-limit com-
parator interrupts the on-time if the sense voltage is
larger than 200mV. When the sense voltage is larger
than 270mV, the secondary gross current-limit com-
parator is activated to discharge the soft-start capaci-
tor. This forces the IC to re-soft-start preventing
inductor current runaway due to the delay of the prima-
ry cycle-by-cycle current limit.
The switch-mode controller also features a low current
shutdown mode, adjustable soft-start, and thermal
shutdown protection.
R1
V_IN
C4
IN
+
-
5V
REFERENCE
MAX16826
Figure 1. Shunt Regulator Block Diagram
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