+

-

1. 708V

5V

DF

Current Mirror

74 PA * DF

V1R7

Vdd

CMPREF

CMP

Vdd

+

-

16 PA

PWM

Ihyst

Current Mirror

Enable

50 PA

CMP

Req

LM27213

www.ti.com

SNVS377A – FEBRUARY 2006 – REVISED MARCH 2013

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 approximately 25A with conventional surface mount power

devices. Hysteretic control assures the fastest possible transient response and a nearly ideal voltage positioning

droop response.

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 Identification) 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 current 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 19 below. The hysteresis current actually consists of four components. The main hysteresis

source is programmed by the current out of the V1R7 reference 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.

Figure 19.

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