LM3424BKBSTEVAL National Semiconductor, LM3424BKBSTEVAL Datasheet - Page 14

• Manufacturer Part Number: LM3424BKBSTEVAL
• Description: BOARD, EVALUATION, FOR LM3424BKBST
• Manufacturer: National Semiconductor

Specifications of LM3424BKBSTEVAL

• Kit Contents: Assembled LM3424 Buck-Boost Evaluation Board, Application Note, LM3424 Datasheet
• Svhc: No SVHC (15-Dec-2010)
• Kit Features: 1A Output, Thermal Foldback, Analogue Dimming, High Frequency
• Mcu Supported Families: LM3424

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If an analog temperature sensor such as the LM94022 is
used, then R
V
Since the NTC is not usually local to the controller, a bypass
capacitor (C
pacitor is used at TSENSE, then a capacitor (C
or greater value should be placed from TREF to GND in order
to ensure the controller does not start-up in foldback. Alter-
natively, a smaller C
function at start-up (see Application Information section).
Thermal foldback is simply analog dimming according to a
specific profile, therefore any method of controlling the differ-
ential voltage between TREF and TSENSE can be use to
analog dim the LED current. The corresponding LED current
for any V
The CSH pin can also be used to analog dim the LED current
by adjusting the current sense voltage (V
mal foldback. There are several different methods to adjust
V
1.
2.
TENSE
SNS
External variable resistance : Adjust a potentiometer
placed in series with R
External variable current source: Source current (0 µA to
I
CSH
using the CSH pin:
will be the direct voltage output of the sensor.
) into the CSH pin to adjust V
DIF
FIGURE 6. Analog Dimming Circuitry
NTC
> 0V is defined:
BIAS
) is suggested from TSENSE to GND. If a ca-
and the NTC are not necessary and
REF
can be used to create a fade-up
CSH
to vary V
SNS
SNS
SNS
.
.
), similar to ther-
REF
) of equal
300857k3
14
In general, analog dimming applications require a lower
switching frequency to minimize the effect of the leading edge
blanking circuit. As the LED current is reduced, the output
voltage and the duty cycle decreases. Eventually, the mini-
mum on-time is reached. The lower the switching frequency,
the wider the linear dimming range.
CSH methods are physically implemented.
Method 1 uses an external potentiometer in the CSH path
which is a simple addition to the existing circuitry. However,
the LEDs cannot dim completely because there is always
some resistance causing signal current to flow. This method
is also susceptible to noise coupling at the CSH pin since the
potentiometer increases the size of the signal current loop.
Method 2 provides a complete dimming range and better
noise performance, though it is more complex. Like thermal
foldback, it simply sources current into the CSH pin, decreas-
ing the amount of signal current that is necessary. This
method consists of a PNP current mirror and a bias network
consisting of an NPN, 2 resistors and a potentiometer
(R
into the CSH pin. A higher resistance value will source more
current into the CSH pin causing less regulated signal current
through R
should be a dual pair PNP for best matching and perfor-
mance. The additional current (I
can be calculated:
The corresponding I
THERMAL SHUTDOWN
The LM3424 includes thermal shutdown. If the die tempera-
ture reaches approximately 165°C the device will shut down
(GATE pin low), until it reaches approximately 140°C where
it turns on again.
ADJ
), where R
HSP
, effectively dimming the LEDs. Q7 and Q8
ADJ
LED
controls the amount of current sourced
for a specific I
ADD
) sourced into the CSH pin
Figure 6
ADD
is:
shows how both