LTC3454
10
3454fa
TYPICAL APPLICATIONS
diode such as an MBRM120T3 or equivalent. Do not use
ordinary rectifi er diodes, since the slow recovery times
will compromise effi ciency.
In applications in which V
IN
is greater than 4V and V
OUT
to
GND short-circuit protection is needed, a Schottky diode
such as MBRM120T3 or equivalent may be used from
GND to SW1 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.
In applications in which V
IN
is greater than 4.5V, a Schottky
diode such as MBRM120T3 or equivalent may be required
from SW1 to V
IN
if the LTC3454 is enabled with an output
voltage already present. The Schottky diode should be
added as close to the pins as possible.
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:
f
LC
Hz
FILTER POLE
OUT
_
•• •
=
π
1
2
where C
OUT
is the output fi lter capacitor.
The output fi lter zero is given by:
f
RC
Hz
FILTER ZERO
ESR OUT
_
•• •
=
π
1
2
where R
ESR
is the capacitor equivalent series resistance.
A troublesome feature in boost mode is the right-half plane
zero (RHP), and is given by:
f
V
ILV
Hz
RHPZ
IN
OUT OUT
=
π
2
2• • • •
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:
f
g
C
UG
m
VC
=
π2• •
where g
m
is the error amp transconductance (typically
1/5.2k) and C
VC
is the external capacitor to GND at the
V
C
pin. For the white LED application, a 0.1μF or greater
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
I
LED
= 3850 [0.8V/(R
ISET1
|| R
ISET2
)]
Since the maximum continuous output current is limited to
1A, this sets a minimum limit on the parallel combination
of R
ISET1
and R
ISET2
equal to
R
MIN
= (R
ISET1
|| R
ISET2
)|
MIN
= 3850(0.8V/1A)
= 3080Ω
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
ISET
resistors and pulsing
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
SET
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.
