Analog Devices LINEAR LTM 8005 Manual page 15

APPLICATIONS INFORMATION
Duty Cycle Considerations
Switching duty cycle is a key variable defining converter
operation; therefore, its limits must be considered when
programming the switching frequency for a particular
application. The fixed minimum on-time and minimum
off-time and the switching frequency define the minimum
and maximum duty cycle of the switch, respectively. When
calculating operating limits, use typical room temperature
values of 320ns and 290ns for minimum on-time and
off-time, respectively.
Setting Input Current Limit
The LTM8005 has a standalone input current sense ampli-
fier to limit the input current. The input current I
Figure 8 is converted to a voltage output at the IVINCOMP
pin. When the IVINCOMP voltage exceeds 1.2V the inter-
nal power switch is turned off, and the converter stops
switching. The input current limit is calculated as follows:
60mV
I
=
IN
R
INSNS
I
IN
V
IN
IVINP
IVINCOMP
Figure 8. Apply a Current Sense Resistor Between
IVINP and IVINN to Limit Input Current
Filter capacitor C shown in Figure 8 filters the voltage
FILT
at the IVINCOMP pin to minimize ripple due to the input
current. C also compensates the input current regula-
FILT
tion loop, and is selected based on the loop response
in addition to the intended voltage ripple on IVINCOMP .
The IVINCOMP pin resistance to ground and C
second pole in the input current regulation loop in addi-
tion to the dominant pole at VC pin. Suggested values for
C of 10nF to 0.1µF will usually provide a second pole
FILT
in the input current regulation loop that results in stable
loop response and is equivalent to the second pole in
shown in
IN
R
INSNS
TO LOAD
IVINN
LTM8005
C
FILT
8005 F08
FILT
For more information
the ISP/ISN regulation loop, which consists of the output
capacitance C
OUT
load. The minimum C
the LTM8005.
Loop Compensation
The LTM8005 uses an internal transconductance error
amplifier whose VC output compensates the control loop.
The external inductor, output capacitor and the compen-
sation resistor and capacitor determine the loop stability.
The inductor and output capacitor are chosen based on
performance, size and cost. The compensation resistor
and capacitor at VC are selected to optimize control loop
response and stability. For typical LED applications, a
10nF compensation capacitor at VC is adequate, and a
series resistor should always be used to increase the slew
rate on the VC pin to maintain tighter regulation of LED
current during fast transients on the input supply to the
converter.
Soft-Start Capacitor Selection
For many applications, it is important to minimize the
inrush current at start-up. The LTM8005 soft-start cir-
cuit significantly reduces the start-up current spike and
output voltage overshoot. The soft-start interval is set by
the soft-start capacitor (C
equation:
T = C • 2V / 28µA
SS SS
A typical value for the soft-start capacitor is 0.1µF. The
soft-start pin voltage reduces the oscillator frequency and
the maximum current in the switch. Soft-start also oper-
ates as fault protection, which forces the converter into
hiccup or latchoff mode. Detailed information is provided
in the Fault Protection: Hiccup Mode and Latchoff Mode
section.
Fault Protection: Hiccup Mode and Latchoff Mode
form a If an LED overcurrent condition, internal INTV
voltage, output short (FB ≤ 0.3V), or thermal limit hap-
pens, the integrated PMOS disconnect switch disconnects
the LED array from the power path, and the integrated
power switching MOSFET is turned off. If the soft-start
pin is charging and still below 1.7V, then it will continue
www.analog.com
LTM8005
and the dynamic resistance of the LED
value of 10nF is integrated into
FILT
) selection according to the
SS
under-
CC
Rev. B
15