LTC3770 Datasheet, PDF (11/24 Pages) Linear Technology – Synchronous Controller with Margining, Tracking and PLL

LTC3770
APPLICATIO S I FOR ATIO
The basic LTC3770 application circuit is shown in
Figure 12. External component selection is primarily de-
termined 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 deter-
mines 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
voltage is about 1.33 times this nominal value.
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 dis-
cussed 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-
resistance. 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) = ^RSENSE
ρ_T
The ρ_T term is a normalization factor (unity at 25°C)
accounting for the significant variation in on-resistance
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
3770f
11