lm27212 National Semiconductor Corporation, lm27212 Datasheet - Page 11

• Manufacturer Part Number: lm27212
• Description: Two-phase Current-mode Hysteretic Buck Controller
• Manufacturer: National Semiconductor Corporation

Operation Description
GENERAL
The LM27212 is a 2-phase current-mode hysteretic buck
regulator controller that meets modern mobile CPU power
requirements.
The LM27212 operates from a 5V supply and generates two
logic signals that can be used to control external MOSFET
drivers. The IC also has two pins (SYNC1 & SYNC2) that
can be used to instruct the external MOSFET drivers to run
synchronous or asynchronous, a feature to enable power
saving operation in Stop CPU or Sleep mode.
CONTROL LOOP
Refer to Figure 1. The core of the control circuit is the error
comparator, which turns off the top switch of a channel when
that channel’s peak inductor current exceeds the current
command. The comparator turns on the top switch of the
other channel when the previous channel’s inductor current
has dropped from its peak value by the preset hysteresis. By
doing this the two channels are turned on and off alternately
with a theoretical phase shift of 180 degrees.
To understand how the current mode works in this topology,
let us assume the hysteresis current (ih) is zero, and the
regulator can switch at infinite frequency. For channel 1,
FIGURE 1. Two-Phase Current-Mode Hysteretic Operation
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since it is switching at infinite frequency, the V- voltage is
always equal to V+ voltage. Since the hysteresis current, ih
is zero, V- is always equal to X1. Therefore the voltage
across RS1 is always equal to that across RR2. In other
words, inductor current in each channel is proportional to the
voltage across RR2. So whenever there is a change in
VCORE, there will be a corresponding but smaller change in
RR2 voltage, which causes a finite change in the inductor
currents. That is how load line programming is achieved.
In reality, the switching frequency is typically a few hundred
kilohertz. The inductor currents therefore has a finite amount
of ripple. The LM27212 sets the inductor ripple current by
alternately forcing a hysteresis current (ih) through RH1 and
RH2. The hysteresis current ih causes a hysteresis voltage
across RH1 and RH2. When CMP1 gets connected to V-
node of the error comparator via the internal mux, ih is
turned on and flows through the RH1 resistor, establishing a
hysteresis voltage across it. The error comparator trips when
V- exceeds V+, at which moment X1 exceeds V+ by the
hysteresis voltage. In other words, roughly half of the ripple
voltage developed across RS1 is equal to the hysteresis
voltage across RH1. After the error comparator trips, the top
switch of Channel 1 is turned off, ih is turned off and L1’s
current starts to decrease. When L1’s current droops to a
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