Texas Instruments LM3404 Evaluation Boards LM3404FSTDIMEV/NOPB LM3404FSTDIMEV/NOPB 사용자 설명서
제품 코드
LM3404FSTDIMEV/NOPB
T
SW
300 ns
V
O(MIN)
= V
IN
x
T
SW
T
SW
- 300 ns
V
O(MAX)
= V
IN
x
T
SW
= 1/f
SW
n
MAX
=
V
F(MAX)
V
O(max)
- 200 mV
D
MAX
=
t
ON
t
ON
+ t
OFF-MIN
V
O(max)
= D
MAX
x V
IN
I
L-MIN
=
0.2
R
SNS
L
-
V
O
x t
SNS
SNVS465F – OCTOBER 2006 – REVISED MAY 2013
AVERAGE LED CURRENT ACCURACY
The COT architecture regulates the valley of
Δ
V
SNS
, the AC portion of V
SNS
. To determine the average LED
current (which is also the average inductor current) the valley inductor current is calculated using the following
expression:
expression:
(3)
In this equation t
SNS
represents the propagation delay of the CS comparator, and is approximately 220 ns. The
average inductor/LED current is equal to I
L-MIN
plus one-half of the inductor current ripple,
Δ
i
L
:
I
F
= I
L
= I
L-MIN
+
Δ
i
L
/ 2
(4)
Detailed information for the calculation of
Δ
i
L
is given in the Design Considerations section.
MAXIMUM OUTPUT VOLTAGE
The 300 ns minimum off-time limits the maximum duty cycle of the converter, D
MAX
, and in turn the maximum
output voltage, V
O(MAX)
, determined by the following equations:
(5)
The maximum number of LEDs, n
MAX
, that can be placed in a single series string is governed by V
O(MAX)
and the
maximum forward voltage of the LEDs used, V
F(MAX)
, using the expression:
(6)
At low switching frequency the maximum duty cycle and output voltage are higher, allowing the LM3404/04HV to
regulate output voltages that are nearly equal to input voltage. The following equation relates switching frequency
to maximum output voltage, and is also shown graphically in the Typical Performance Characteristics section:
regulate output voltages that are nearly equal to input voltage. The following equation relates switching frequency
to maximum output voltage, and is also shown graphically in the Typical Performance Characteristics section:
(7)
MINIMUM OUTPUT VOLTAGE
The minimum recommended on-time for the LM3404/04HV is 300 ns. This lower limit for t
ON
determines the
minimum duty cycle and output voltage that can be regulated based on input voltage and switching frequency.
The relationship is determined by the following equation, shown on the same graphs as maximum output voltage
in the Typical Performance Characteristics section:
The relationship is determined by the following equation, shown on the same graphs as maximum output voltage
in the Typical Performance Characteristics section:
(8)
HIGH VOLTAGE BIAS REGULATOR
The LM3404/04HV contains an internal linear regulator with a 7V output, connected between the VIN and the
VCC pins. The VCC pin should be bypassed to the GND pin with a 0.1 µF ceramic capacitor connected as close
as possible to the pins of the IC. VCC tracks VIN until VIN reaches 8.8V (typical) and then regulates at 7V as
VIN increases. Operation begins when VCC crosses 5.25V.
VCC pins. The VCC pin should be bypassed to the GND pin with a 0.1 µF ceramic capacitor connected as close
as possible to the pins of the IC. VCC tracks VIN until VIN reaches 8.8V (typical) and then regulates at 7V as
VIN increases. Operation begins when VCC crosses 5.25V.
12
Copyright © 2006–2013, Texas Instruments Incorporated
Product Folder Links: