lm27213mtd National Semiconductor Corporation, lm27213mtd Datasheet - Page 13

• Manufacturer Part Number: lm27213mtd
• Description: Single Phase Hysteretic Buck Controller
• Manufacturer: National Semiconductor Corporation

Applications Information
The LM27213 is a single phase current-mode hysteretic
controller intended for controlling a power supply for a low
voltage CPU core. It is capable of currents up to approxi-
mately 25A with conventional surface mount power devices.
Hysteretic control assures the fastest possible transient re-
sponse and a nearly ideal voltage positioning droop re-
sponse.
THEORY OF OPERATION
The LM27213 controls the inductor ripple current on a cycle
by cycle basis. Several reference voltages are available
depending on the mode of operation selected. There is an
internal DAC that gets programmed via 6 VID (Voltage Iden-
tification) bits. In addition, there are several inputs that allow
separate references to be selected in various “sleep modes”.
An internal MUX selects the reference to be used by the
control loop. A softstart function controls the rate at which the
selected reference is allowed to ramp up at turn on. There is
a cycle by cycle current limit loop as well as over voltage
protection.
CONTROL LOOP OPERATION
The main regulator loop is a current mode hysteretic design
that maintains control over the buck inductor’s peak-peak
ripple current. A small hysteresis current is forced to flow
through resistor RH which is connected between the CMP
pin and the left side of the current sense resistor. When the
high-side switch is on, this current flows into the pin forcing
CMP below CMPREF. As the inductor current increases, the
voltage at CMP rises. The error comparator turns off the
high-side switch and turns on the low-side switch when the
inductor current exceeds the demand. When the high-side
switch turns off the hysteresis current is reversed and cur-
rent is sourced from the CMP pin. The error comparator now
allows the inductor current to decay until the new threshold is
crossed. Refer to Figure 1 below. The hysteresis current
actually consists of four components. The main hysteresis
source is programmed by the current out of the V1R7 refer-
ence pin. The total divider resistance on this pin controls the
magnitude of this current which is mirrored and sent to the
CMP pin. This source will only be active when the high-side
switch is on. In addition, there’s a small correction current
that varies as a function of duty factor. Its magnitude is
approximately 74µA*DF, where DF is the duty factor. At
typical operating duty factors, it will be around 7µA and in the
same direction as the main hysteresis current. This current
will flow at all times. The tail current from the current sense
comparator also needs to be accounted for. This 16µA flows
from the CMP pin when PWM is high, and from the CMPREF
pin when PWM is low. Its direction is opposite that of the
hysteresis current source and so subtracts from the total
hysteresis current. The final contribution to the hysteresis
current is the 50uA that is sourced continuously and serves
as the “off” hysteresis. It is recommended that approximately
100µA be programmed through the V1R7 pin. At this level
the “on” and “off” currents are approximately symmetrical
around zero.
13
The current through the V1R7 pin is simply 1.708V divided
by Req. Therefore the effective divider resistance should be
approximately 17kΩ. The hysteresis current when the high-
side switch is on (assuming a roughly 10% duty factor) is
100µA +7µA -16µA - 50µA = 41µA. When the high-side
switch is off Ihyst is only 7µA -50µA = - 43µA. It is the
difference between these two levels that controls the pk-pk
inductor current or:
Note that the correction current (74µA *DF) does not appear
in this equation. It serves only to move the output voltage
slightly as a function of duty factor to correct for offsets that
are inherent in the topology.
Figure 2 shows a higher level picture of the control loop. The
reference that the CMP voltage is compared to is the voltage
at the CMPREF pin. Assume that the CMPREF pin is simply
tied to a fixed reference voltage (R2 open). The control loop
would force the peak voltage at point A minus the hysteresis
voltage to equal the reference level. Since the on and off
hysteresis currents are symmetrical around zero, the aver-
age voltage at point A is therefore equal to the reference
voltage. There will be a voltage droop as a function of load
(load line) equal to the value of Rsense, the current sense
resistor. Adding resistors R1 and R2 allows this load line
slope to be increased without raising the value of the sense
resistor. The voltage across R2 will equal the voltage across
the sense resistor, Rs. So, if R1 = R2 the load line will be
twice Rs. Algebraically,
When the high-side switch has been turned off, the 16µA
comparator input bias current now flows out of the CMPREF
pin through the parallel combination of R1 and R2. This
results in a small increase in the reference voltage (about
1mV with typical values) and will reduce the size of the
hysteresis band by an equal amount.
LL = R
∆I
hyst
FIGURE 1.
= I
s
x (1+R1/R2)
V1R7
-16µA
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