LTC3454 Linear Technology, LTC3454 Datasheet - Page 10

• Manufacturer Part Number: LTC3454
• Description: 1A Synchronous Buck-Boost High Current LED Driver
• Manufacturer: Linear Technology

LTC3454
APPLICATIO S I FOR ATIO
ordinary rectifi er diodes, since the slow recovery times
will compromise effi ciency.
In applications in which V
GND short-circuit protection is needed, a Schottky diode
such as MBRM12OT3 or equivalent may be used from
SW1 to GND and/or a 2Ω/1nF series snubber from SW1
to GND. The Schottky diode should be added as close
to the pins as possible. Neither of these is required for
shorted LED protection.
Closing the Feedback Loop
The LTC3454 incorporates voltage mode PWM control.
The control to output gain varies with operation region
(Buck, Boost, Buck/Boost), but is usually no greater
than 15. The output fi lter exhibits a double pole response
given by:
where C
The output fi lter zero is given by:
where R
A troublesome feature in Boost mode is the right-half
plane zero (RHP), and is given by:
The loop gain is typically rolled off before the RHP zero
frequency.
A simple Type I compensation network can be incorporated
to stabilize the loop but at a cost of reduced bandwidth
and slower transient response. To ensure proper phase
margin, the loop is required to be crossed over a decade
before the LC double pole.
The unity-gain frequency of the error amplifi er with the
Type I compensation is given by:
10
f
f
f
f
FILTER POLE
FILTER ZERO
UG
RHPZ
=
ESR
OUT
2 • •
=
_
_
is the capacitor equivalent series resistance.
2 • •
is the output fi lter capacitor.
π
g
m
π
C
=
=
VC
I
OUT
2
2
U
V
• •
• •
IN
π
π
2
• •
L V
IN
R
U
1
ESR
is greater than 4V and V
L C
1
OUT
•
•
OUT
C
Hz
OUT
W
Hz
Hz
U
OUT
to
where g
1/5.2k) and C
V
capacitor value is recommended.
Maximum LED Current
As described in the Operation section, the output LED
current with both enable pins logic high is equal to
Since the maximum continuous output current is limited to
1A, this sets a minimum limit on the parallel combination
of R
Although the LTC3454 can safely provide this current
continuously, the external LED may not be rated for this
high a level of continuous current. Higher current levels
are generally reserved for pulsed applications, such as
LED camera fl ash. This is accomplished by programming
a high current with one of the R
the appropriate enable pin.
Varying LED Brightness
Continuously variable LED brightness control can be
achieved by interfacing directly to one or both of the I
pins. Figure 3 shows four such methods employing a
voltage DAC, a current DAC, a simple potentiometer or a
PWM input. It is not recommended to control brightness
by PWMing the enable pins directly as this will toggle
the LTC3454 in and out of shutdown and result in erratic
operation.
LED Failure Modes
If the LED fails as an open circuit, the safety amplifi er takes
control of the regulation loop to prevent V
The V
The safety amplifi er has no effect on loop regulation at
V
If the LED fails as a short-circuit, the current limiting
circuitry detects this condition and limits the peak input
current to a safe level.
C
OUT
I
R
pin. For the white LED application, a 0.1µF or greater
LED
ISET1
MIN
less than 5.15V.
OUT
= 3850 [0.8V/(R
= (R
= 3080Ω
m
and R
threshold at which this occurs is about 5.15V.
is the error amp transconductance (typically
ISET1
VC
ISET2
is the external capacitor to GND at the
|| R
equal to
ISET2
ISET1
)|
MIN
|| R
ISET
= 3850(0.8V/1A)
ISET2
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resistors and pulsing
)]
OUT
runaway.
SET
3454f