TRACK

FB

LM3495

Master

Power

Supply

SNS

LM3495

www.ti.com

SNVS410F – FEBRUARY 2006 – REVISED APRIL 2013

APPLICATIONS INFORMATION

THEORY OF OPERATION

The LM3495 is an advanced, current mode PWM synchronous controller. Unlike traditional peak current mode

controllers which sense the current while the high-side FET is on, the LM3495 senses current while the low-side

FET is on. The LM3495 then emulates the peak current waveform and uses that information to regulate the

output voltage. High-side ON pulses as low as 50 ns are possible to achieve low duty cycle operation. The

LM3495 therefore enjoys both excellent line transient response and the ability to regulate low output voltages

from high input voltages.

START UP

The LM3495 will begin to operate when the COMP/SD pin is open-circuited and the voltage at the VIN pin has

exceeded 2.6V. Once these two conditions have been met an internal soft start begins and lasts for 400

switching cycles. When soft start is complete the converter enters steady state operation. Current limit is enabled

during soft start to protect against a short circuit at the output.

START UP INTO OUTPUT PRE-BIAS

If the output capacitor of the LM3495 regulator has been charged up to some pre-bias level before the converter

is enabled, the soft start will ramp the output voltage from the pre-bias level up to the target output voltage

without ever discharging the output capacitor. Note that the pre-bias voltage must not be greater than the target

output voltage of the LM3495, otherwise the LM3495 will pull the pre-bias supply down during steady state

operation. A zero-cross comparator prevents the current in the inductor from reversing during soft start and

prevents discharge of the output capacitor through the low-side FET. In FPWM mode, once soft-start is complete

the zero-cross threshold decreases over 16 cycles and then is disabled, allowing the converter to sink current at

the output if needed.

The LM3495 contains an internal N-FET with an on-resistance of approximately 500

Ω connected between the

SNS and PGND pins. When the converter is disabled, this FET is turned on to discharge the output capacitor in

a controlled fashion. If the LM3495 is used in a system with a pre-bias at the output the power supply providing

the pre-bias must be able to supply enough current for the 500

Ω load that the internal FET creates.

TRACKING

The LM3495 can track the output of a master power supply during soft start by connecting a resistor divider to

the TRACK pin (Figure 30). In this way, the output voltage slew rate of the LM3495 will be controlled by the

master supply for loads that require precise sequencing. Because the output of the master supply is divided

down, the output voltage of the LM3495 must be lower than the voltage of the master supply in order to track

properly. When the tracking function is not being used, the TRACK pin should be connected directly to the VLIN5

pin.

Figure 30. Tracking Circuit

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