LTC3736-1 Datasheet, PDF (10/28 Pages) Linear Technology – Dual 2-Phase, No RSENSE Synchronous Controller with Spread Spectrum

LTC3736-1
U
OPERATIO (Refer to Functional Diagram)
Main Control Loop
The LTC3736-1 uses a current mode architecture with the
two controllers operating 180 degrees out of phase.
During normal operation, the top external P-channel power
MOSFET is turned on when the clock for that channel sets
the RS latch, and turned off when the current comparator
(I_CMP) resets the latch. The peak inductor current at which
I_CMP resets the RS latch is determined by the voltage on
the I_TH pin, which is driven by the output of the error
amplifier (EAMP). The V_FB pin receives the output voltage
feedback signal from an external resistor divider. This
feedback signal is compared to the internal 0.6V reference
voltage by the EAMP. When the load current increases, it
causes a slight decrease in V_FB relative to the 0.6V refer-
ence, which in turn causes the I_TH voltage to increase until
the average inductor current matches the new load cur-
rent. While the top P-channel MOSFET is off, the bottom
N-channel MOSFET is turned on until either the inductor
current starts to reverse, as indicated by the current
reversal comparator, I_RCMP, or the beginning of the next
cycle.
Shutdown, Soft-Start and Tracking Start-Up
(RUN/SS and TRACK Pins)
The LTC3736-1 is shut down by pulling the RUN/SS pin
low. In shutdown, all controller functions are disabled and
the chip draws only 9µA. The TG outputs are held high (off)
and the BG outputs low (off) in shutdown. Releasing
RUN/SS allows an internal 0.7µA current source to charge
up the RUN/SS pin. When the RUN/SS pin reaches 0.65V,
the LTC3736-1’s two controllers are enabled.
The start-up of V_OUT1 is controlled by the LTC3736-1’s
internal soft-start. During soft-start, the error amplifier
EAMP compares the feedback signal V_FB1 to the internal
soft-start ramp (instead of the 0.6V reference), which rises
linearly from 0V to 0.6V in about 1ms. This allows the
output voltage to rise smoothly from 0V to its final value,
while maintaining control of the inductor current.
The 1ms soft-start time can be increased by connecting
the optional external soft-start capacitor C_SS between the
RUN/SS and SGND pins. As the RUN/SS pin continues to
rise linearly from approximately 0.65V to 1.3V (being
charged by the internal 0.7µA current source), the EAMP
regulates the V_FB1 proportionally linearly from 0V to 0.6V.
The start-up of V_OUT2 is controlled by the voltage on the
TRACK pin. When the voltage on the TRACK pin is less
than the 0.6V internal reference, the LTC3736-1 regulates
the V_FB2 voltage to the TRACK pin instead of the 0.6V
reference. Typically, a resistor divider on V_OUT1 is con-
nected to the TRACK pin to allow the start-up of V_OUT2 to
“track” that of V_OUT1. For one-to-one tracking during start-
up, the resistor divider would have the same ratio as the
divider on V_OUT2 that is connected to V_FB2.
Light Load Operation
The LTC3736-1 operates in PWM pulse skipping mode at
light loads. In this mode, the current comparator I_CMP may
remain tripped for several cycles and force the external P-
channel MOSFET to stay off for the same number of cycles.
The inductor current is not allowed to reverse (discontinu-
ous operation). This mode exhibits low output ripple as
well as low audio noise and reduced RF interference, while
providing high light load efficiency.
Spread Spectrum Operation
Switching regulators can be particularily troublesome in
applications where electromagnetic interference (EMI) is
a concern. Switching regulators operate on a cycle-by-
cycle basis to transfer power to an output. In most cases,
the frequency of operation is either fixed or is a constant
based on the output load. This method of conversion
creates large components of noise at the frequency of
operation (fundamental) and multiples of the operating
frequency (harmonics). Figures 1a and 1b depict the
output noise spectrum of a conventional buck switching
converter (1/2 of LTC3736-1 with spread spectrum opera-
tion disabled) with V_IN = 5V, V_OUT = 2.5V and I_OUT = 2A.
Unlike conventional buck converters, the LTC3736-1’s
internal oscillator is designed to produce a clock pulse
whose frequency is randomly varied between 450kHz and
580kHz. This has the benefit of spreading the switching
noise over a range of frequencies, thus significantly reduc-
ing the peak noise. Figures 1c and 1d show the output
noise spectrum of the LTC3736-1 (with spread spectrum
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