LTC2952
13
2952fb
OPERATION
The LTC2952 is designed to simplify applications requiring
management of multiple power sources. The three main
features of the part are: pushbutton control, ideal diode
PowerPath controllers and system monitoring. The Block
Diagram on the previous page shows the part divided into
its main functional blocks.
The pushbutton detect block is responsible for debouncing
any pushbutton event on the PB pin. Note that the ON and
OFF debounce times can be configured independently by
using two separate capacitors on the ONT and OFFT pins
respectively. A valid pushbutton on event will set the EN
pin high impedance and a valid off event will drive the
EN pin low.
In a typical application the EN pin is tied to the shutdown
pin of a DC/DC converter. Therefore, by toggling the EN pin,
the pushbutton pin has a direct control over the enabling/
disabling of an external DC/DC converter. This control of
system turn-on/off is done in a graceful manner which
ensures proper system power-up and power-down.
The ideal diode drivers regulate two external P-channel
MOSFETs to achieve low loss switchover between two DC
sources. Each driver regulates the gate of the PFET such
that the voltage drop across its source and drain is 20mV.
When the load current is larger than the PFET ability to
deliver such current with a 20mV drop across its source
and drain, the voltage at the gate clamps at V
G(ON)
and
the PFET behaves like a fixed value resistor.
Besides providing ideal diode PowerPath controllers and
control of system power turn-on/off, the LTC2952 also
provides system monitoring function via the VM, WDE,
RST and PFI, PFO pins. The voltage monitoring (VM) and
the watchdog (WDE) input pins determine the state of the
RST output with 200ms reset time and 1.6s watchdog
time. The PFI and PFO pins are the input and output of an
accurate comparator that can be used as an early power
fail monitor.
The KILL, M1 and M2 pins are the inputs to accurate
comparators with 0.5V threshold. The outputs of these
comparators interact with the logic block to alter the ideal
diode PowerPath controllers and the pushbutton control
behavior. Specifically, the KILL input provides the system
with a capability to turn off system power at any point
during operation. The M1 and M2 pins are mode pins
that configure the part to have different behaviors in the
PowerPath switchover of the two DC sources.
Figure 1 shows the four different typical configurations of
the LTC2952. In configuration A, both of the ideal-diode
PowerPath controllers are always enabled which results
in an automatic switchover between the two DC sources.
configuration C is similar to A except for the pushbutton
input which now controls both the EN pin and the ideal
diode PowerPath controllers.
In configurations B and D, M2 is used as a voltage monitor.
In B, when the M2 input is above its threshold the primary
ideal diode PowerPath is disabled. In D, M2 needs to be
above threshold before PB has control over the EN pin
and the ideal diode PowerPath controllers. Furthermore
in D, the rising and falling edges on M2 are interpreted as
turn-on and turn-off commands, respectively.
