LTC3858 Datasheet, PDF (14/38 Pages) Linear Technology – Low IQ, Dual 2-Phase Synchronous Step-Down Controller

LTC3858
Operation (Refer to the Functional Diagram)
between the two internal controllers, as summarized in
Table 1. The phases are calculated relative to the zero
degrees phase being defined as the rising edge of the top
gate driver output of controller 1 (TG1).
Table 1
V_PHASMD
GND
Floating
INTV_CC
CONTROLLER 2 PHASE
180°
180°
240°
CLKOUT PHASE
60°
90°
120°
Output Overvoltage Protection
An overvoltage comparator guards against transient over-
shoots as well as other more serious conditions that may
overvoltage the output. When the V_FB pin rises by more
than 10% above its regulation point of 0.800V, the top
MOSFET is turned off and the bottom MOSFET is turned
on until the overvoltage condition is cleared.
Power Good (PGOOD1 and PGOOD2) Pins
Each PGOOD pin is connected to an open drain of an
internal N-channel MOSFET. The MOSFET turns on and
pulls the PGOOD pin low when the corresponding V_FB pin
voltage is not within ±10% of the 0.8V reference voltage.
The PGOOD pin is also pulled low when the corresponding
RUN pin is low (shut down). When the V_FB pin voltage
is within the ±10% requirement, the MOSFET is turned
off and the pin is allowed to be pulled up by an external
resistor to a source no greater than 6V.
Theory and Benefits of 2-Phase Operation
Why the need for 2-phase operation? Up until the 2‑phase
family, constant frequency dual switching regulators
operated both channels in phase (i.e., single phase
operation). This means that both switches turned on at
the same time, causing current pulses of up to twice the
amplitude of those for one regulator to be drawn from the
input capacitor and battery. These large amplitude current
pulses increased the total RMS current flowing from the
input capacitor, requiring the use of more expensive input
capacitors and increasing both EMI and losses in the input
capacitor and battery.
With 2-phase operation, the two channels of the dual
switching regulator are operated 180 degrees out of phase.
This effectively interleaves the current pulses drawn by the
switches, greatly reducing the overlap time where they add
together. The result is a significant reduction in total RMS
input current, which in turn allows less expensive input
capacitors to be used, reduces shielding requirements for
EMI and improves real world operating efficiency.
Figure 2 compares the input waveforms for a representa-
tive single-phase dual switching regulator to the LTC3858
2-phase dual switching regulator. An actual measurement of
the RMS input current under these conditions shows that
2-phase operation dropped the input current from 2.53A_RMS
to 1.55A_RMS. While this is an impressive reduction in itself,
remember that the power losses are proportional to I_RMS2,
meaning that the actual power wasted is reduced by a
5V SWITCH
20V/DIV
3.3V SWITCH
20V/DIV
INPUT CURRENT
5A/DIV
INPUT VOLTAGE
500mV/DIV
I_IN(MEAS) = 2.53A_RMS
I_IN(MEAS) = 1.55A_RMS
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Figure 2. Input Waveforms Comparing Single-Phase (a) and 2-Phase (b) Operation for Dual Switching Regulators
Converting 12V to 5V and 3.3V at 3A Each. The Reduced Input Ripple with the 2-Phase Regulator Allows
Less Expensive Input Capacitors, Reduces Shielding Requirements for EMI and Improves Efficiency
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