Instrukcja obsługi Microchip HV9910BDB1

Microchip nieskategoryzowany HV9910BDB1

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Supertex inc.
Supertex inc.
www.supertex.com
Doc.# DSDB-HV9910BDB3
A032813
HV9910BDB3
General Description
The HV9910BDB3 demoboard is a high current LED driver
designed to drive one LED or two LEDs in series at currents
up to 1.0A from a 10 – 30VDC input. The demoboard uses
Supertex’s HV9910B Universal LED driver IC to drive a
buck converter.
The HV9910BDB3 can be congured to operate in either a
constant frequency mode (for driving a single LED) or in a
constant o-time mode (for driving two LEDs).
The output current can be adjusted in two ways – either
with linear dimming using the onboard potentiometer or
with PWM dimming by applying a TTL – compatible square
wave signal at the PWMD terminal. Using linear dimming,
the output current of the HV9910DB1 can be lowered to
about 0.01A (note: zero output current can be obtained
only by PWM dimming).
Connection Diagram
Low Voltage, High Current,
LED Driver Demoboard
Specications
Parameter Value
Input voltage 10 - 30VDC
Output voltage -
constant frequency mode 2.0 - 4.5V
Output voltage -
constant o-time mode 4.0 - 8.0V
Maximum output current 1.0A ± 10%
Output current ripple (typ) 20% (peak-peak)
Efciency (@ 12V input) 86% (for one LED)
93% (for two LEDs)
Open LED protection yes
Output short circuit protection no
Dimensions 48.2mm X 29.0mm
Connections
1. Input Connection - Connect the input DC voltage
between VIN and GND terminals of connector J1 as shown
in the connection diagram.
2. Output Connection - Connect the LEDs between
LED+ (anode of LED string) and LED- (cathode of LED
string) of connector J2.
a. If the load is one LED, short the RT and FREQ
terminals of connector J4 using a jumper.
b. If the load is two LEDs, short the RT and OFFT
terminals of connector J4 using a jumper.
3. PWM Dimming Connection
a. If no PWM dimming is required, short PWMD and
VDD terminals of connector J3.
b. If PWM dimming is required, connect the TTL-
compatible PWM sourc between PWMD and GND
terminals of connector J3. The recommended PWM
dimming frequency is ≤ 1.0kHz.
+
Short for constant
frequency mode
+
-
Short for
constant o-time mode
2
Supertex inc.
www.supertex.com
Doc.# DSDB-HV9910BDB3
A032813
HV9910BDB3
Frequently Asked Questions
1. Why does the demoboard have two operating
modes?
Constant frequency mode limits the maximum output
voltage to less then 50% of the minimum input voltage.
So, in this case, if we use only the constant frequency
mode, the maximum output voltage will have to be less
than 5V. Constant o-time mode removes this limita-
tion and allows the output voltage become higher. How-
ever, in order to achieve reasonable noise immunity and
to limit the switching frequency variation over the input
voltage range, it is not recommended to operate the
HV9910DB3 with the output voltage exceeding 80% of
the input voltage, even in the constant o-time mode.
Please refer to application note AN-H50 on the Supertex
website for more details.
2. If the minimum input voltage in my application is high-
er (say 20V), does that mean I can drive a 9V LED string
in the constant frequency mode or an 16V LED string in
the constant o-time mode using the demoboard?
Although a larger LED string can be driven using the
demoboard in these conditions, the demoboard will not
be able to drive the LED at 1A.
The HV9910B is a constant peak current controller. The
average LED current is equal to the peak current set
(using the sense resistor) minus one-half of the ripple
current in the inductor.
Higher output voltages lead to larger ripple current val-
ues, which will reduce the maximum LED current the
board can deliver.
3. How can I compute the maximum LED current the
demoboard can deliver if I use a higher input voltage
and a higher LED string voltage?
See table below:
Parameters Minimum input voltage
Maximum LED string voltage
Switching frequency (constant frequency mode)
O-Time (constant off-time mode)
HV9910B CS threshold voltage
Sense Resistor
Inductor
= VIN,MIN
= VO,MAX
= fS
= TOFF
= VCS
= RCS
= L
(100kHz)
(5.1μs)
(0.25V)
(0.22Ω)
(220μH)
Constant O-Time Mode Constant Frequency Mode
V
O,MAX
• T
OFF
L
Δl =
ILED = –
VCS
R
CS
Δl
2
VO, MAX
VIN,MIN
1 –
Maximum Switching Frequency =
{
{
TOFF
VO, MAX
VIN,MIN
1 –
{
L • fS
VO,MAX •
I
LED = –
VCS
RCS
Δl
2
Δl =
3
Supertex inc.
www.supertex.com
Doc.# DSDB-HV9910BDB3
A032813
HV9910BDB3
Frequently Asked Questions (cont.)
4. If the constant o-time mode allows a wider LED volt-
age range, why not use that mode exclusively? Why do
we need the constant frequency mode?
Although the constant o-time mode allows the
demoboard to operate at a higher output voltage, the
LED ripple current is directly proportional to the output
voltage in this mode. This makes it difcult to get a good
load regulation of the LED current in the constant o-
time mode with a wide variation in the LED string voltage
(in this case it will be a 1:4 variation). At lower LED volt-
age values, the ripple will be lower and the LED current
would be higher.
By switching between the two modes depending on the
load, we can get a better current accuracy without hav-
ing to adjust the LD voltage or the sense resistor.
5. Why is the efciency of the demoboard higher with a
load of two LEDs compared to a single LED load?
Losses in the HV9910BDB3 occur due mainly due to
two factors:
a. Conduction losses in the FET and diode
b. Switching losses in the FET
Switching losses are dependent on the switching fre-
quency, input voltage and total parasitic capacitance at
the node. At higher switching frequencies, the switching
losses are higher.
Conduction losses are dependent on the duty cycle.
Since the voltage drop on the FET is smaller than the
voltage drop on the diode (the on-resistance of the FET
is very small), the higher the duty cycle, the smaller is
the conduction loss. Please note that we are ignoring
the losses in the inductor, which will be identical in both
cases.
Also, efciency = POUT / PIN = POUT / (POUT + losses) = 1/ (1
+ losses/POUT), where POUT is the output power and PIN
is the input power. So, if the output power is higher, the
xed switching losses are a smaller fraction of the output
power and thereby the efciency is higher.
Comparing the operation of the converter in both modes
at 12V input for this particular demoboard, the following
are the dierences:
a. Output power is higher with 2 LEDs as the load
b. Switching frequency in the constant o-time
mode is 55kHz, whereas it is 100kHz in the constant
frequency mode
c. Duty cycle of operation is about higher in the con-
stant o-time mode by a factor of 2 than in the con-
stant frequency mode
All the above factors favor the higher load voltage and thus
the demoboard has a higher efciency when the load is larg-
er.
6. Why are the LED current rise and fall times during
PWM dimming dierent when the load changes from
one LED to two LEDs?
The LED current rise time is directly proportional to VIN
- VOUT and the fall time is proportional to VOUT (where VIN
is the input voltage and VOUT is the output voltage). Since
VOUT
is higher with two LEDs, the rise time will be larger
and the fall time will be smaller.
Load Regulation
(@ VIN = 12V)
0
2
4
6
8
10
2 4
6 8
Load ltage (V)Vo
Change in current (%)
Constant Of me Modef-Ti
Constant o-time mode
Constant Frequency Mode Constant O-time Mode
With mode change

Specyfikacje produktu

Marka: Microchip
Kategoria: nieskategoryzowany
Model: HV9910BDB1

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