LTC3626_15 Datasheet, PDF (16/28 Pages) Linear Technology – 20V, 2.5A Synchronous Monolithic Step-Down Regulator with Current and Temperature Monitoring

LTC3626
Applications Information
results. Furthermore, this supply is intended only to supply
additional DC load currents as desired and not intended to
regulate large transient or AC behavior as this may impact
LTC3626 operation.
Boost Capacitor
The boost capacitor, C_BST, on the Functional Diagram
is used to create a voltage rail above the applied input
voltage, V_IN. Specifically, the boost capacitor is charged
to a voltage equal to approximately INTV_CC each time the
bottom power MOSFET is turned on. The charge on this
capacitor is then used to supply the required transient
current during the remainder of the switching cycle. When
the top MOSFET is turned on, the BOOST pin voltage will
be equal to approximately V_IN + 3.3V. For most applica-
tions a 0.1μF ceramic capacitor will provide adequate
performance.
Minimum Off-Time/On-Time Considerations
The minimum off-time is the smallest amount of time that
the LTC3626 requires to turn on the bottom power MOS-
FET, trip the current comparator and turn off the power
MOSFET. This time is typically 40ns. For the controlled
on-time current mode control architecture, the minimum
off-time limit imposes a maximum duty cycle of:
DC_MAX = 1 – (f • t_OFF(MIN))
where f is the switching frequency and t_OFF(MIN) is the
minimum off-time. If the maximum duty cycle is surpassed,
due to a dropping input voltage for example, the output
will drop out of regulation. The minimum input voltage to
avoid this dropout condition is:
( ) ^{VIN(MIN) = 1–}
V_OUT
f • t_OFF(MIN)
Output Voltage Programming
The LTC3626 will adjust the output voltage such that V_FB
equals the reference voltage of 0.6V according to:
V_OUT
=
0.6V ^{⎛⎝⎜}1+
R1⎞
R2 ⎠⎟
The desired output voltage is set by the appropriate selec-
tion of resistors R1 and R2 as shown in Figure 2. Choosing
large values for R1 and R2 will result in improved efficiency
but may lead to undesired noise coupling or phase margin
reduction due to stray capacitance at the FB node. Care
should be taken to route the FB line away from any noise
source, such as the SW or BOOST lines.
To improve the frequency response of the main control
loop a feedforward capacitor, C_F, may be used as shown
in Figure 2.
FB
LTC3626
SGND
V_OUT
R1
C_F
R2
3626 F02
Figure 2. Optional Feedforward Capacitor
Users should consider reducing the LTC3626 operating
frequency for applications that may violate the minimum
off-time if constant regulation is required.
Conversely, the minimum on-time is the smallest dura-
tion of time in which the top power MOSFET can be in
its “on” state. This time is typically 20ns. In continuous
mode operation, the minimum on-time limit imposes a
minimum duty cycle of:
DC_MIN = (f • t_ON(MIN))
where t_ON(MIN) is the minimum on-time. As the equation
shows, reducing the operating frequency will alleviate the
minimum duty cycle constraint.
In rare cases in which the LTC3626’s minimum duty
cycle is surpassed, the output voltage will still remain in
regulation, however the switching frequency will be lower
than its programmed value. This is an acceptable result in
many applications, so high switching frequencies may be
used in the design without fear of severe consequences.
As the sections on Inductor and Capacitor Selection show,
high switching frequencies allow the use of smaller board
components, thus reducing the footprint of the applica-
tion circuit.
3626fa
16
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