LTC3604 Linear Technology, LTC3604 Datasheet - Page 15

• Manufacturer Part Number: LTC3604
• Description: 2.5A 15V Monolithic Synchronous Step-Down Regulator
• Manufacturer: Linear Technology

a soft-start function by connecting a capacitor from the
TRACK/SS pin to ground. The relationship between output
rise time and TRACK/SS capacitance is given by:
A default internal soft-start timer forces a minimum soft-
start time of 400μs by overriding the TRACK/SS 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/SS 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/SS pin accuracy.
Output Power Good
The PGOOD output of the LTC3604 is driven by a 15Ω
(typical) open-drain pull-down device. This device will
be turned off once the output voltage is within ±5% (typi-
cal) of the target regulation point allowing the voltage at
PGOOD to rise via an external pull-up resistor (100k typi-
cal). 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
the output voltage must be within the target regulation
window 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).
Effi ciency Considerations
The percent effi ciency of a switching regulator is equal to
the output power divided by the input power times 100%.
APPLICATIONS INFORMATION
t
SS
= 430,000 × C
FB
> 0.48V). Once the output reaches this
TRACK/SS
OUT
transient events. As a result,
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 LTC3604: 1) I
and quiescent current loss, 3) transition losses and other
system losses.
1. I
2. The internal LDO supplies the power to the INTV
% 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:
The R
obtained from the Typical Performance Characteristics
curves. Thus to obtain I
The total power loss here is the sum of the switching
losses and quiescent current losses from the control
circuitry.
2
R loss is calculated from the DC resistances of the
R
“I
SW
2
R LOSS” = I
VOLTAGE
DS(ON)
PGOOD
= (R
for both the top and bottom MOSFETs can be
DS(ON)TOP
Figure 4. PGOOD Pin Behavior
–8%
OUT
NOMINAL OUTPUT
SW
–5%
2
)(DC) + (R
· (R
, and external inductor, R
2
0%
R loss:
2
R loss, 2) switching losses
SW
5%
+ R
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DS(ON)BOT
L
8%
)
LTC3604
V
OUT
3604 F04
DS(ON)
)(1 – DC)
15
and the
CC
L
rail.
. In
3604f