LM3402MRX National Semiconductor, LM3402MRX Datasheet - Page 11

• Manufacturer Part Number: LM3402MRX
• Description: LM3402/LM3402HV 0.5A Constant Current Buck Regulator for Driving High Power LEDs; Package: PSOP; No of Pins: 8; Qty per Container: 2500; Container: Reel
• Manufacturer: National Semiconductor

The LM3402/02HV regulators should be operated in contin-
uous conduction mode (CCM), where inductor current stays
positive throughout the switching cycle. During steady-state
operationin the CCM, the converter maintains a constant
switching frequency, which can be selected using the follow-
ing equation:
AVERAGE LED CURRENT ACCURACY
The COT architecture regulates the valley of ΔV
portion of V
is also the average inductor current) the valley inductor cur-
rent is calculated using the following expression:
In this equation t
CS comparator, and is approximately 220 ns. The average
inductor/LED current is equal to I
ductor current ripple, Δi
Detailed information for the calculation of Δi
Design Considerations section.
MAXIMUM OUTPUT VOLTAGE
The 300 ns minimum off-time limits on the maximum duty cy-
cle of the converter, D
voltage V
V
F
= forward voltage of each LED, n = number of LEDs in
O(MAX)
SNS
. To determine the average LED current (which
is determined by the following equations:
SNS
I
F
represents the propagation delay of the
= I
MAX
L
L
:
= I
, and in turn ,the maximum output
series
L-MIN
L-MIN
+ Δi
L
plus one-half of the in-
/ 2
FIGURE 1. Comparator and One-Shot
L
is given in the
SNS
, the AC
11
The maximum number of LEDs, n
a single series string is governed by V
mum forward voltage of the LEDs used, V
expression:
At low switching frequency the maximum duty cycle and out-
put voltage are higher, allowing the LM3402/02HV to regulate
output voltages that are nearly equal to input voltage. The
following equation relates switching frequency to maximum
output voltage.
MINIMUM OUTPUT VOLTAGE
The minimum recommended on-time for the LM3402/02HV is
300 ns. This lower limit for t
cycle and output voltage that can be regulated based on input
voltage and switching frequency. The relationship is deter-
mined by the following equation:
20192105
ON
determines the minimum duty
MAX
, that can be placed in
O(MAX)
F(MAX)
and the maxi-
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, using the