LTC3770_15 Datasheet, PDF (11/24 Pages) Linear Technology – Fast No RSENSE Step-Down Synchronous Controller with Margining, Tracking and PLL

LTC3770
APPLICATIONS INFORMATION
The basic LTC3770 application circuit is shown in Figure 12.
External component selection is primarily determined by
the maximum load current and begins with the selection
of the sense resistance and power MOSFET switches. The
LTC3770 uses either a sense resistor or the on-resistance
of the synchronous power MOSFET for determining the
inductor current. The desired amount of ripple current
and operating frequency largely determines the inductor
value. Finally, C_IN is selected for its ability to handle the
large RMS current into the converter and C_OUT is chosen
with low enough ESR to meet the output voltage ripple
and transient specification.
Maximum Sense Voltage and V_RNG Pin
Inductor current is determined by measuring the voltage
across a sense resistance that appears between the SENSE^–
(PGND on G package) and SENSE⁺ (SW on G package)
pins. The maximum sense voltage is set by the voltage
applied to the V_RNG pin and is equal to approximately
(0.133)V_RNG. The current mode control loop will not allow
the inductor current valleys to exceed (0.133)V_RNG/R_SENSE.
In practice, one should allow some margin for variations
in the LTC3770 and external component values and a good
guide for selecting the sense resistance is:
R_SENSE
=
10
V_RNG
• I_OUT(MAX)
An external resistive divider from INTV_CC can be used
to set the voltage of the V_RNG pin between 0.5V and 2V
resulting in nominal sense voltages of 50mV to 200mV.
Additionally, the V_RNG pin can be tied to SGND or INTV_CC
in which case the nominal sense voltage defaults to 50mV
or 200mV, respectively. The maximum allowed sense volt-
age is about 1.33 times this nominal value. However tying
the V_RNG pin to ground is not recommended because the
current comparator offset is now a bigger proportion of
the total sense voltage.
Connecting the SENSE⁺ and SENSE^– Pins
The LTC3770 comes in UH and G packages. The UH package
IC can be used with or without a sense resistor. When
using a sense resistor, place it between the source of the
bottom MOSFET, M2, and PGND. Connect the SENSE⁺ and
SENSE^– pins to the top and bottom of the sense resistor.
Using a sense resistor provides a well defined current
limit, but adds cost and reduces efficiency. Alternatively,
one can eliminate the sense resistor and use the bottom
MOSFET as the current sense element by simply connecting
the SENSE⁺ pin to the SW pin and SENSE^– pin to PGND.
This improves efficiency, but one must carefully choose
the MOSFET on-resistance as discussed below.
Power MOSFET Selection
The LTC3770 requires two external N-channel power
MOSFETs, one for the top (main) switch and one for the
bottom (synchronous) switch. Important parameters for
the power MOSFETs are the breakdown voltage V_(BR)DSS,
threshold voltage V_(GS)TH, on-resistance R_DS(ON), reverse
transfer capacitance C_RSS and maximum current I_DS(MAX).
The gate drive voltage is set by the 5V INTV_CC supply.
Consequently, logic-level threshold MOSFETs must be used
in LTC3770 applications. If the input voltage is expected
to drop below 5V, then sub-logic level threshold MOSFETs
should be considered.
When the bottom MOSFET is used as the current sense
element, particular attention must be paid to its on-resis-
tance. MOSFET on-resistance is typically specified with
a maximum value R_DS(ON)(MAX) at 25°C. In this case,
additional margin is required to accommodate the rise in
MOSFET on-resistance with temperature:
R_DS(ON)(MAX)
=
R_SENSE
ρ_T
2.0
1.5
1.0
0.5
0
–50
0
50
100
150
JUNCTION TEMPERATURE (°C)
3770 F01
Figure 1. R_DS(ON) vs Temperature
3770fc
11