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LTC3601

3601f

APPLICATIONS INFORMATION

a soft-start function by connecting a capacitor from the

TRACK pin to ground. The relationship between output

rise time and TRACK capacitance is given by:

tSS = 430,000 × CTRACK/SS

A default internal soft-start timer forces a minimum soft-

start time of 400μs by overriding the TRACK pin input

during this time period. Hence, capacitance values less

than approximately 1000pF will not signiﬁcantly affect

soft-start behavior.

When using the TRACK pin, the regulator defaults to

Burst Mode operation until the output exceeds 80% of

its ﬁnal value (VFB > 0.48V). Once the output reaches this

voltage, the operating mode of the regulator switches to

the mode selected by the MODE/SYNC pin as described

above. During normal operation, if the output drops below

10% of its ﬁnal value (as it may when tracking down, for

instance), the regulator will automatically switch to Burst

Mode operation to prevent inductor saturation and improve

TRACK pin accuracy.

Output Power Good

The PGOOD output of the LTC3601 is driven by a 15Ω

(typical) open-drain pull-down device. This device will be

turned off once the output voltage is within 5% (typical) of

the target regulation point allowing the voltage at PGOOD

to rise via an external pull-up resistor (100k typical). If

the output voltage exits a 8% (typical) regulation window

around the target regulation point the open-drain output

will pull down with 15Ω output resistance to ground,

thus dropping the PGOOD pin voltage. A ﬁlter time of

40μs (typical) acts to prevent unwanted PGOOD output

changes during VOUT transient events. As a result, the

output voltage must be within the target regulation win-

dow of 5% for 40μs before the PGOOD pin is pulled high.

Conversely, the output voltage must exit the 8% regulation

window for 40μs before the PGOOD pin pulls to ground

(see Figure 4).

Efﬁciency Considerations

The percent efﬁciency of a switching regulator is equal to

the output power divided by the input power times 100%.

It is often useful to analyze individual losses to determine

what is limiting the efﬁciency and which change would

produce the most improvement. Percent efﬁciency can

be expressed as:

% Efﬁciency = 100% – (L1 + L2 + L3 +…)

where L1, L2, etc. are the individual loss terms as a per-

centage of input power.

Although all dissipative elements in the circuit produce

losses, three main sources account for the majority of the

losses in the LTC3601: 1) I2R loss, 2) switching losses

and quiescent current loss, 3) transition losses and other

system losses.

1. I2R loss is calculated from the DC resistances of the

internal switches, RSW, and external inductor, RL. In

continuous mode, the average output current will

ﬂow through inductor L but is “chopped” between the

internal top and bottom power MOSFETs. Thus, the

series resistance looking into the SW pin is a function

of both the top and bottom MOSFET’s RDS(ON) and the

duty cycle (DC) as follows:

RSW = (RDS(ON)TOP)(DC) + (RDS(ON)BOT)(1 – DC)

PGOOD

VOLTAGE

VOUT

–8%

–5%

3601 F04

NOMINAL OUTPUT

Figure 4. PGOOD Pin Behavior

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