LTC3788_15 Datasheet, PDF (15/32 Pages) Linear Technology – 2-Phase, Dual Output Synchronous Boost Controller

LTC3788
Operation
MOSFET turns on. There are two considerations to keep
the BOOST supply at the required bias level. During
start-up, if the bottom MOSFET is not turned on within
100µs after UVLO goes low, the bottom MOSFET will be
forced to turn on for ~400ns. This forced refresh gener-
ates enough BOOST-SW voltage to allow the top MOSFET
ready to be fully enhanced instead of waiting for the initial
few cycles to charge up. There is also an internal charge
pump that keeps the required bias on BOOST. The charge
pump always operates in both forced continuous mode
and pulse-skipping mode. In Burst Mode operation, the
charge pump is turned off during sleep and enabled when
the chip wakes up. The internal charge pump can normally
supply a charging current of 55µA.
Applications Information
The Typical Application on the first page is a basic
LTC3788 application circuit. LTC3788 can be configured
to use either inductor DCR (DC resistance) sensing or a
discrete sense resistor (R_SENSE) for current sensing. The
choice between the two current sensing schemes is largely
a design trade-off between cost, power consumption and
accuracy. DCR sensing is becoming popular because it
does not require current sensing resistors and is more
power-efficient, especially in high current applications.
However, current sensing resistors provide the most
accurate current limits for the controller. Other external
component selection is driven by the load requirement,
and begins with the selection of R_SENSE (if R_SENSE is used)
and inductor value. Next, the power MOSFETs are selected.
Finally, input and output capacitors are selected.
SENSE⁺ and SENSE^– Pins
The SENSE⁺ and SENSE ^– pins are the inputs to the cur-
rent comparators. The common mode input voltage range
of the current comparators is 2.5V to 38V. The current
sense resistor is normally placed at the input of the boost
controller in series with the inductor.
The SENSE⁺ pin also provides power to the current
comparator. It draws ~200µA during normal operation.
There is a small base current of less than 1µA that flows
into the SENSE ^– pin. The high impedance SENSE ^– input
to the current comparators allow accurate DCR sensing.
Filter components mutual to the sense lines should be
placed close to the LTC3788, and the sense lines should
run close together to a Kelvin connection underneath the
current sense element (shown in Figure 1). Sensing cur-
rent elsewhere can effectively add parasitic inductance
and capacitance to the current sense element, degrading
the information at the sense terminals and making the
programmed current limit unpredictable. If DCR sensing
is used (Figure 2b), sense resistor R1 should be placed
close to the switching node, to prevent noise from coupling
into sensitive small-signal nodes.
TO SENSE FILTER,
NEXT TO THE CONTROLLER
V_IN
INDUCTOR OR R_SENSE
3788 F01
Figure 1. Sense Lines Placement with
Inductor or Sense Resistor
Sense Resistor Current Sensing
A typical sensing circuit using a discrete resistor is shown
in Figure 2a. R_SENSE is chosen based on the required
output current.
The current comparator has a maximum threshold
V_SENSE(MAX). When the ILIM pin is grounded, floating or
tied to INTV_CC, the maximum threshold is set to 50mV,
75mV or 100mV, respectively. The current comparator
threshold sets the peak of the inductor current, yielding
a maximum average output current, I_MAX, equal to the
peak value less half the peak-to-peak ripple current, ∆I_L.
To calculate the sense resistor value, use the equation:
R_SENSE
=
V_{SENSE(MAX )}
I_MAX
+
∆I_L
2
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