LM5037MTSLASHNOPB datasheet(14/40 Pages) TI

14 / 40 page

CFF

Vin

SLOPE

PROPORTIONAL

TO Vin

RAMP

COMP

Gate

Drive

CLK

RFF

VIN

LM5037

SNVS578D – NOVEMBER 2008 – REVISED MAY 2015

www.ti.com

Feature Description (continued)

Figure 12. Feed-Forward Voltage Mode Configuration

7.3.9 Current Mode

The LM5037 device can be configured for current mode control by injecting a signal representative of primary

current into the RAMP pin. One way to achieve this is shown in Figure 13. Filter components Rfilter and Cfilter are

used to filter leading edge noise spikes. The signal at the CS pin is thus a ramp on a pedestal. The pedestal

corresponds to the continuous conduction current in the transformer at the beginning of an OUTA or OUTB

conduction cycle. The R-C circuit (RSlope and CSlope), shown in Figure 13, tied to VREF adds an additional ramp to

the current sense signal. This additional ramp signal, known as slope compensation, is required to avoid

instabilities at duty cycles above 50% (25% per phase). The compensated RAMP signal consists of two parts,

the primary current signal and the slope compensation. The compensated RAMP signal is compared to the error

signal by the PWM comparator to control the duty cycle of the outputs. The RAMP capacitor and CS capacitor

are reset through internal discharge FETs. The on-resistance (RDS(on)) of RAMP discharge FET is 5 Ω (typical);

this ensures fast discharge of the RAMP reset capacitor. Any dc voltage source can be used in place of VREF to

generate the slope compensation ramp.

The timing diagram shown in Figure 14 depicts the current mode waveforms and relative timing. When OUTA or

OUTB is enabled, the signal at the RAMP pin consists of the CS pin signal (current ramp on a pedestal) plus the

slope compensation ramp (dotted lines). When OUTA or OUTB is turned off, the primary current component is

absent but the voltage at the RAMP pin continues to rise due to slope compensation component until the end of

the clock period, after which it is reset by the RAMP discharge FET. A component selection example is explained

in detail in the Application and Implementation section.

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LM5037MT/NOPB Datenblatt(PDF) 14 Page - Texas Instruments