LTC3601 Linear Technology, LTC3601 Datasheet - Page 15

• Manufacturer Part Number: LTC3601
• Description: 15V Monolithic Synchronous Step-Down Regulator
• Manufacturer: Linear Technology

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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:
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 signifi cantly affect
soft-start behavior.
When using the TRACK pin, the regulator defaults to
Burst Mode operation until the output exceeds 80% of
its fi nal value (V
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 fi nal 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 fi lter time of
40μs (typical) acts to prevent unwanted PGOOD output
changes during V
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).
APPLICATIONS INFORMATION
t
SS
= 430,000 × C
FB
OUT
> 0.48V). Once the output reaches this
TRACK/SS
transient events. As a result, the
Effi ciency Considerations
The percent effi ciency 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 effi ciency and which change would
produce the most improvement. Percent effi ciency can
be expressed as:
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) I
and quiescent current loss, 3) transition losses and other
system losses.
1. I
% Effi ciency = 100% – (L1 + L2 + L3 +…)
internal switches, R
continuous mode, the average output current will
fl ow 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 R
duty cycle (DC) as follows:
2
R loss is calculated from the DC resistances of the
R
SW
VOLTAGE
PGOOD
= (R
DS(ON)TOP
Figure 4. PGOOD Pin Behavior
–8%
NOMINAL OUTPUT
SW
–5%
)(DC) + (R
, and external inductor, R
0%
2
R loss, 2) switching losses
5%
DS(ON)BOT
8%
LTC3601
V
OUT
3601 F04
DS(ON)
)(1 – DC)
15
and the
L
. In
3601f