lt3743 Linear Technology Corporation, lt3743 Datasheet - Page 17

• Manufacturer Part Number: lt3743
• Description: High Current Synchronous Step-down Led Driver With Three-state Control
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
MOSFET Selection for the Switched Output Capacitors
The MOSFETs used for the switched-output capacitor need
to also handle the maximum regulated current while the
capacitor is charged. The output drivers on the PWMGH
and PWMGL pins have a pull-up impedance of 3.2Ω and
a pull-down impedance of 1.75Ω. This provides adequate
gate drive for 30nC MOSFETs without the need for an
additional gate driver. If the LED forward resistance and
the difference between the two regulated currents is large
enough, then two MOSFETs are required to prevent the body
diode of the MOSFET from conducting and discharging
the capacitor for the high current state. Figure 9 shows
the output capacitor for the high current regulation state
discharged with the body diode when a single MOSFET
is used. Figure 10 shows the application circuit with a
drain-to-drain confi guration for the high current output
capacitor. In this confi guration, the body diode of the up-
per MOSFET blocks conduction and prevents discharge
of the high current output capacitor.
Figure 10. With a Drain-to-Drain Confi guration, the Body
Diode of the Top FET Blocks the Current Path That Would
Discharge the High Current Output Capacitor
LT3743
LT3743
PWMGH
PWMGL
PWMGH
PWMGL
Figure 9. Body Diode of High Current FET
Discharges the Output Capacitor
I
I
CTRL_H
I
CTRL_L
I
CTRL_H
CTRL_L
0V
5V
OFF
= 20A
= 1A
= 20A
= 1A
3.8V
3.8V
ON
3743 F09
3743 F10
3.04V
3.04V
V
R
V
R
F
D
F
D
= 3V
= 40mΩ
= 3V
= 40mΩ
If the voltage between the low state and the high state is
very large (greater than the threshold of the MOSFET) then
the capacitor may once again be discharged. To account for
this, choose a MOSFET that has a threshold greater than
the voltage difference. If the voltage difference exceeds
1.5V, use the circuit shown in Figure 11. The circuit shown
will keep the capacitor from discharging to a voltage dif-
ference of approximately 2V + V
Board and Interconnect Inductance
The board and interconnect inductance from the output
capacitors to the load also determine the rate of change
in load (LED) current. The rate of change in load current
will be:
where dI
V
at the high level, and V
inductor is regulated at the low state. When measuring the
LED current do not use a current probe. The core material
used in most probes adds inductance and slows the rise
time of the LED current. Instead, when measuring the
current, use a sense resistor.
HIGH
dI
dt
L
is the output voltage when the inductor is regulated
=
Figure 11. Application for Large Differences
in Regulated Currents
L
V
/dt is the rate of change in the load current,
HIGH
V
LT3743
CC_INT
L
PWMGH
PWMGL
BOARD
–
I
CTRL_H
V
I
CTRL_L
LOW
3.01k
= 20A
LOW
= 1A
2V
is the output voltage when the
3743 F11
2k
TH
.
V
R
F
D
= 3V
= 200mΩ
LT3743
17
3743f