Datasheet

LT3743
17
3743fd
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 the PWM 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 configuration for the high current output
capacitor. In this configuration, the body diode of the up-
per MOSFET blocks conduction and prevents discharge
of the high current output capacitor.
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
TH
.
APPLICATIONS INFORMATION
Figure 9. Body Diode of High Current FET
Discharges the Output Capacitor
Figure 10. With a Drain-to-Drain Configuration, the Body
Diode of the Top FET Blocks the Current Path That Would
Discharge the High Current Output Capacitor
Figure 11. Application for Large Differences
in Regulated Currents
LT3743
OFF
0V
3.8V
5V
PWMGH
3743 F09
PWMGL
3.04V
V
F
= 3V
R
D
= 40mΩ
I
CTRL_L
= 1A
I
CTRL_H
= 20A
ON
LT3743
3.8V
PWMGH
3743 F10
PWMGL
3.04V
V
F
= 3V
R
D
= 40mΩ
I
CTRL_L
= 1A
I
CTRL_H
= 20A
LT3743
3.01k
2k
2V
V
CC_INT
PWMGH
3743 F11
PWMGL
V
F
= 3V
R
D
= 200mΩ
I
CTRL_L
= 1A
I
CTRL_H
= 20A
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:
dI
L
dt
=
V
HIGH
– V
LOW
L
BOARD
where dI
L
/dt is the rate of change in the load current,
V
HIGH
is the output voltage when the inductor is regulated
at the high level, and V
LOW
is the output voltage when the
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.