Datasheet

C
FF
Vin
SLOPE
PROPORTIONAL
TO Vin
RAMP
COMP
Gate
Drive
1V
CLK
R
FF
V
IN
LM25037
LM25037, LM25037-Q1
www.ti.com SNVS572D –JULY 2008–REVISED MARCH 2013
Figure 17. Feed-Forward Voltage Mode Configuration
CURRENT MODE
The LM25037 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 18. Filter components R
filter
and C
filter
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 (R
Slope
and C
Slope
), shown in Figure 18, tied to V
REF
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 R
DS(ON)
of RAMP discharge FET is 5Ω (typical); this ensures fast discharge of the
RAMP reset capacitor. Any dc voltage source can be used in place of V
REF
to generate the slope compensation
ramp.
The timing diagram shown in Figure 19 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 Information section.
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