LM3495MTC datasheet(12/26 Pages) NSC

12 / 26 page

Applications Information

THEORY OF OPERATION

The LM3495 is an advanced, current mode PWM synchro-

nous controller. Unlike traditional peak current mode control-

lers 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 op-

eration. 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 volt-

age 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 compara-

tor 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 1). 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.

One way to use the tracking feature is to design the tracking

resistor divider so that the master supply output voltage

OUT1) and the LM3495 output voltage (VOUT2) both rise

together and reach their target values at the same time. For

this case, the equation governing the values of the tracking

divider resistors R

T1 and RT2 is:

The above equation is set equal to 0.65V in order to ensure

that the final value of the track pin voltage exceeds the

reference voltage of the LM3495, and this 50 mV offset will

cause the LM3495 output voltage to reach regulation slightly

before the master supply. A value of 10 k

Ω 1% is recom-

mended for R

T2 as a good compromise between high preci-

sion and low quiescent current through the divider. If the

master supply voltage V

OUT1 is 5V, for example, then the

value of R

T1 needed to give the two supplies identical soft

start times would be 1.5 k

Ω 1%. A timing diagram for this

example, the equal soft start time case, is shown in Figure 2.

Alternatively, the tracking feature can be used to create

equal slew rates between the output voltages of the LM3495

and the master supply. This method ensures that the output

voltage of the LM3495 always reaches regulation before the

20169929

FIGURE 1. Tracking Circuit

20169931

FIGURE 2. Tracking with Equal Soft Start Time

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