Sample and

Hold DC Level

1V/A

RAMP

TON

tON

CRAMP

(10 P x (VIN ± VOUT) + 50 P) x

13

LM5575-Q1

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SNOSB23F – OCTOBER 2008 – REVISED JULY 2019

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Device Functional Modes (continued)

7.4.4 Ramp Generator

The ramp signal used in the pulse-width modulator for current-mode control is typically derived directly from the

buck switch current. This switch current corresponds to the positive slope portion of the output inductor current.

Using this signal for the PWM ramp simplifies the control-loop-transfer function to a single pole response and

provides inherent input voltage feed-forward compensation. The disadvantage of using the buck switch current

signal for PWM control is the large leading edge spike due to circuit parasitics that must be filtered or blanked.

Also, the current measurement may introduce significant propagation delays. The filtering, blanking time, and

propagation delay limit the minimum achievable pulse width. In applications where the input voltage may be

relatively large in comparison to the output voltage, controlling small pulse widths and duty cycles is necessary

for regulation. The LM5575-Q1 uses a unique ramp generator, which does not actually measure the buck switch

current but rather reconstructs the signal. Reconstructing or emulating the inductor current provides a ramp

signal to the PWM comparator that is free of leading edge spikes and measurement or filtering delays. The

current reconstruction is comprised of two elements; a sample and hold DC level and an emulated current ramp.

Figure 12. Composition of Current Sense Signal

The sample and hold DC level shown in Figure 12 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 and 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 Equation 2:

IRAMP = (10 µA × (VIN – VOUT)) + 50 µA

(2)

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

CRAMP can be selected from Equation 3:

CRAMP = L × 10

–5

where

•

L is the value of the output inductor in Henrys

(3)