LTC3870_15 Datasheet, PDF (9/22 Pages) Linear Technology – PolyPhase Step-Down Slave Controller for LTC3880/LTC3883 with Digital Power System Management

LTC3870
Operation
Main Control Loop
The LTC3870 is a constant frequency, current mode
step-down slave controller for parallel operation with the
LTC3880/LTC3883. During normal operation, each top
MOSFET is turned on when the clock for that channel
sets the RS latch, and turned off when the main current
comparator, ICMP, resets the RS latch. The peak inductor
current at which ICMP resets the RS latch is controlled
by the voltage on the I_TH pin, which is tied directly to the
corresponding I_TH pin of the master controllers (LTC3880/
LTC3883). When the load current increases, LTC3880/
LTC3883 master controllers drive and increase the I_TH
voltage, which in turn cause the peak current in the cor-
responding slave channels to increase, until the average
inductor current matches the new load current. After the
top MOSFET has been turned off, the bottom MOSFET is
turned on until the beginning of the next cycle in Continu-
ous Conduction Mode (CCM) or until the inductor current
starts to reverse, as indicated by the reverse current
comparator I_REV, in Discontinuous Conduction Mode
(DCM). The LTC3870 slave controllers DO NOT regulate
the output voltage but regulate the current in each channel
for current sharing with master controllers. Output voltage
regulation is achieved through the voltage feedback loops
in master controllers.
INTV_CC/EXTV_CC Power
Power for the top and bottom MOSFET drivers and most
other internal circuitry is derived from the INTV_CC pin.
Normally an internal 5.0V linear regulator supplies INTV_CC
power from V_IN. In high V_IN applications, if a high effi-
ciency external voltage source is available for the EXTV_CC
pin, another internal 5.0V linear regulator is enabled and
supplies INTV_CC power from EXTV_CC. To enable the linear
regulator driven by the EXTV_CC pin, V_IN has to be higher
than 6.5V and EXTV_CC pin voltage has to be higher than
4.8V. Do not exceed 14V on the EXTV_CC pin.
Each top MOSFET driver is biased from the floating
bootstrap capacitor C_B, which normally recharges during
each off cycle through an external diode when the top
MOSFET turns off. If the input voltage V_IN decreases to
a voltage close to V_OUT, the loop may enter dropout and
attempt to turn on the top MOSFET continuously. The
dropout detector detects this and forces the top MOSFET
off for about one-twelfth of the clock period plus 100ns
every three cycles to allow C_B to recharge. However, it is
recommended that a load be present or the IC operates
at low frequency during the drop-out transition to ensure
C_B is recharged.
Start-Up and Shutdown (RUN0, RUN1)
The two channels of the LTC3870 can independently
start up and shut down using the RUN0 and RUN1 pins.
Pulling either of these pins below 1.4V shuts down the
control circuits for that channel. During shutdown, both
TG and BG are pulled down to turn off the external power
MOSFETs. Pulling either of these pins above 2V enables
the corresponding channel and internal circuits. During
startup, the RUN0/RUN1 pins are actively pulled down
until the INTV_CC voltage passes the under-voltage lockout
threshold of 4V. For multiphase parallel operation, the
RUN0/RUN1 pins have to be connected and driven by
the RUN pins of the master controller. Do not exceed the
Absolute Maximum Rating of 6V on these pins.
The start-up of each channel’s output voltage V_OUT is
controlled and programmed by the master controller. After
the RUN pins are released, the master controller drives the
output based on the programmed delay time and rise time,
and the slave controller LTC3870 just follows the master
to supply equivalent current to the output during startup.
Light Load Current Operation (Discontinuous
Conduction Mode, Continuous Conduction Mode)
The LTC3870 can be set to operate either in Discontinuous
Conduction Mode (DCM) or forced Continuous Conduc-
tion Mode (CCM). To select forced Continuous Mode of
operation, tie the MODE pin to a DC voltage above 2V
(e.g., INTV_CC). To select discontinuous conduction mode
of operation, tie the MODE pin to a DC voltage below 1.4V
(e.g., SGND). In forced continuous operation, the induc-
tor current is allowed to reverse at light loads or under
large transient conditions. The peak inductor current is
determined by the voltage on the I_TH pin. In this mode,
the efficiency at light loads is lower than in discontinu-
ous Mode operation. However, continuous mode has the
advantages of lower output ripple and less interference
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