LTC3626_15 Datasheet, PDF (20/28 Pages) Linear Technology – 20V, 2.5A Synchronous Monolithic Step-Down Regulator with Current and Temperature Monitoring

LTC3626
Applications Information
MODE/SYNC Operation
The MODE/SYNC pin is a multipurpose pin allowing both
mode selection and operating frequency synchronization.
Connecting this pin to INTV_CC enables Burst Mode operation
for superior efficiency at low load currents at the expense
of slightly higher output voltage ripple. When the MODE/
SYNC pin is pulled to ground, forced continuous mode
operation is selected creating the lowest output voltage
ripple at the expense of light load efficiency.
The LTC3626 will detect the presence of an external clock
signal on the MODE/SYNC pin and synchronize the internal
oscillator to the phase and frequency of the incoming clock.
The presence of an external clock will place the LTC3626
into forced continuous mode operation.
Output Voltage Tracking and Soft-Start
The LTC3626 allows the user to control the output volt-
age ramp rate by means of the TRACK/SS pin. From 0V
to 0.6V the TRACK/SS pin will override the internal refer-
ence input to the error amplifier forcing regulation of the
feedback voltage to that seen at the TRACK/SS pin. When
the voltage at the TRACK/SS pin rises above 0.6V, tracking
is disabled and the feedback voltage will be regulated to
the internal reference voltage.
The voltage at the TRACK/SS pin may be driven from an
external source, or alternatively, the user may leverage the
internal 1.4µA (typical) pull-up current on TRACK/SS to
implement 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:
t_SS = 430,000 • C_TRACK/SS
A default internal soft-start timer forces a minimum soft-
start time of 400µs (typical) by overriding the TRACK/
SS pin input during this time period. Hence, capacitance
values less than approximately 1000pF will not significantly
affect soft-start behavior.
When using the TRACK/SS pin, the regulator defaults to
Burst Mode operation until the output exceeds 80% of
its final value (V_FB > 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/SS pin accuracy.
Output Power Good
The PGOOD output of the LTC3626 is driven by a 20Ω
(typical) open-drain pull-down device. This pin will become
high impedance once the output voltage is within 5%
(typical) of the target regulation point allowing the volt-
age at PGOOD to rise via an external pull-up resistor. If
the output voltage exits a 8% (typical) regulation window
around the target regulation point, the open-drain output
will pull down to ground, thereby dropping the PGOOD
pin voltage. A filter time of 40μs (typical) acts to prevent
unwanted PGOOD output changes during V_OUT transient
events. As a result, 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).
NOMINAL OUTPUT
PGOOD
VOLTAGE
–8% –5% 0% 5%
V_OUT
8%
3626 F04
Figure 4. PGOOD Pin Behavior
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.
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