Sample and

Hold DC Level

0.5V/A

RAMP

TON

tON

CRAMP

(5P x (VIN ± VOUT) + 25P) x

LM25005

SNVS411C – JANUARY 2006 – REVISED MARCH 2013

www.ti.com

Figure 13. Composition of Current Sense Signal

The Sample and Hold DC level illustrated in Figure 13. is derived from a measurement of the re-circulating

Schottky diode anode current. The re-circulating diode anode should be connected to the IS pin. The diode

current flows through an internal current sense resistor between the IS and PGND pins. The voltage level across

the sense resistor is sampled and held just prior to the onset of the next conduction interval of the buck switch.

The diode current sensing and sample & hold provide the dc level of the reconstructed current signal. The

positive slope inductor current ramp is emulated by an external capacitor connected from the RAMP pin to AGND

and an internal voltage controlled current source. The ramp current source that emulates the inductor current is a

function of the Vin and Vout voltages per the following equation:

IRAMP = (5µ x (Vin – Vout)) + 25µA

(2)

Proper selection of the RAMP capacitor depends upon the selected value of the output inductor. The value of

CRAMP can be selected from: CRAMP = L x 10

-5, where L is the value of the output inductor in Henrys. With this

value, the scale factor of the emulated current ramp will be approximately equal to the scale factor of the dc level

sample and hold ( 0.5 V / A). The CRAMP capacitor should be located very close to the device and connected

directly to the pins of the IC (RAMP and AGND).

For duty cycles greater than 50%, peak current mode control circuits are subject to sub-harmonic oscillation.

Sub-harmonic oscillation is normally characterized by observing alternating wide and narrow pulses at the switch

node. Adding a fixed slope voltage ramp (slope compensation) to the current sense signal prevents this

oscillation. The 25µA of offset current provided from the emulated current source adds some fixed slope to the

ramp signal. In some high output voltage, high duty cycle applications, additional slope may be required. In these

applications, a pull-up resistor may be added between the VCC and RAMP pins to increase the ramp slope

compensation.

For VOUT > 7.5V:

Calculate optimal slope current, IOS = VOUT x 5µA/V.

For example, at VOUT = 10V, IOS = 50µA.

Install a resistor from the RAMP pin to VCC:

RRAMP = VCC / (IOS - 25µA)

12

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