lt3740 Linear Technology Corporation, lt3740 Datasheet - Page 8

• Manufacturer Part Number: lt3740
• Description: Wide Operating Range, Valley Mode, No Rsense Synchronous Step-down Controller
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
LT3740
Current Sensing Range
Inductor current is determined by measuring the voltage
across a sense resistance – either the on-resistance of the
bottom MOSFET or an external sensing resistor. The
maximum current sense threshold has three steps that are
selected by the RANGE pin. The current sense threshold
voltage without slope compensation is shown in Table 1.
This is the value for high duty cycle operation.
Table 1. Current Sensing Thresholds
RANGE Pin
Ground
Open
V
Slope Compensation
The LT3740 has a compensation slope to stabilize the
constant-frequency valley mode operation. The slope com-
pensation signal increases with the bottom gate duty
cycle, which results in a current sense threshold voltage
change with duty cycle as shown in the figure in Typical
Performance Characteristics. The three current limit
levels correspond to three compensation slopes.
The compensation slope needs to overcome the difference
between the up and down slope of the inductor current to
avoid sub-harmonic oscillation. Maximum compensation
slope is required for high input voltages, where the duty
cycle is small. The compensation slope can only be
selected by the RANGE pin. In the case of insufficient
compensation slope, the inductor ripple current or the
sensing resistance needs to be reduced.
8
IN
U
U
Current Sensing Threshold
50mV
80mV
105mV
W
U
Reverse Current Limit
Because the LT3740 operates in forced continuous mode
when the feedback voltage is higher than 720mV, the
inductor current can go negative on occasion, such as
light load, shutting down with a slow SHDN signal, large
load step-down transient response, or the output voltage
being pulled up by some other power supply. The LT3740
has a reverse current comparator to limit the reverse
current. During the on-time of the bottom MOSFET, when
(V
triggered and turns off the bottom MOSFET.
When operated under light load, the inductor current goes
negative every cycle. The design of the inductor current
ripple and the sensing resistor need to ensure that the
reverse current comparator is not triggered during normal
operation.
Power MOSFET Selection
The LT3740 requires two external N-channel power
MOSFETs, one for the top switch and one for the bottom
switch. Important parameters for the power MOSFETs are
the breakdown voltage V
on-resistance R
and maximum current I
When the bottom MOSFET is used as the current sense
element, particular attention must be paid to the initial
variation, the gate-source voltage effect and the tempera-
ture characteristics of its on-resistance. MOSFET on-
resistance decreases as the gate-source voltage increases.
The change of BGDP voltage could affect the bottom
MOSFET gate voltage. Refer to the MOSFET datasheet for
the MOSFET on-resistance corresponding to certain gate
voltage.
MOSFET on-resistance is typically specified with a
maximum value R
additional margin is required to accommodate the rise in
MOSFET on-resistance with temperature:
SN
R
DS(ON)
+)–(V
= R
SN
–) reaches 40mV, the comparator is
SENSE
DS(ON)
/ρ
DS(ON)
, reverse transfer capacitance C
T
(BR)DSS
DS(MAX)
at 25°C. In this case,
, threshold voltage V
.
(GS)TH
3740fa
RSS
,