Synchronization (SYNC) and

lation is improved because the PWM duty cycle of the aux-

iliary converter is immediately corrected, independent of the

delays of the voltage regulation loop.

The recommended SYNC input current range is 50µA to

150µA. The SYNC pin resistor (R

set the SYNC current (I

phase signal amplitude, V

that I

change in phase signal amplitude. The SYNC pin resistor is

therefore:

R

Ω

Once I

RAMP signal amplitude may be programmed by selecting

the proper RAMP pin capacitor value. The recommended

peak amplitude of the RAMP waveform is 1V to 1.75V. The

C

amplitude with the nominal phase signal voltage and pulse

width.

C

Where

C

I

T

V

For example,

Main channel output = 3.3V. Phase signal maximum ampli-

tude = 12V. Phase signal frequency = 250kHz

•

Set I

tude (12V):

I

Ω) = 12V /

(R

Ω)

R

Ω = 77.5kΩ

•

T

(3.3V/12V) / 250kHz = 1.1µs

•

Assume desired V

•

C

/ 1.5V

•

C

Error Amplifier and Soft-Start (FB,

CO, & COMP, SS)

An internal wide bandwidth error amplifier is provided within

the LM5115 for voltage feedback to the PWM controller. The

amplifier’s inverting input is connected to the FB pin. The

output of the auxiliary converter is regulated by connecting a

voltage setting resistor divider between the output and the

FB pin. Loop compensation networks are connected be-

tween the FB pin and the error amplifier output (COMP). The

amplifier’s non-inverting input is internally connected to the

SS pin. The SS pin is biased at 0.75V by a resistor divider

connected to the internal 1.27V bandgap reference. When

the VCC voltage is below the UVLO threshold, the SS pin is

discharged to ground. When VCC rises and exceeds the

positive going UVLO threshold (4.25V), the SS pin is re-

leased and allowed to rise. If an external capacitor is con-

nected to the SS pin, it will be charged by the internal resistor

divider to gradually increase the non-inverting input of the

error amplifier to 0.75V. The equivalent impedance of the SS

resistor divider is nominally 60k

Ω which determines the

charging time constant of the SS capacitor. During start-up,

the output of the LM5115 converter will follow the exponen-

tial equation:

VOUT(t) = VOUT(final) x (1 - exp(-t/R

Where

Rss = internal resistance of SS pin (60k

Ω)

Css = external Soft-Start capacitor

VOUT(final) = regulator output set point

The initial

∆v/ ∆t of the output voltage is VOUT(final) / Rss x

Css and VOUT will be within 1% of the final regulation level

after 4.6 time constants or when t = 4.6 x Rss x Css.

Pull-up current for the error amplifier output is provided by an

internal 300µA current source. The PWM threshold signal at

the COMP pin can be controlled by either the open drain

error amplifier or the open drain current amplifier connected

through the CO pin to COMP. Since the internal error ampli-

20134913

FIGURE 3. Line Feed-forward Waveforms

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