Datasheet

t
SS
=
C6 x 2.5V
12.3 PA
C1 =
I
O
x t
ON
'V
LM2695
SNVS413A –JANUARY 2006–REVISED APRIL 2013 www.ti.com
If the capacitor used for C2 does not have sufficient ESR, R3 is added in series as shown in Typical Application
Circuit and Block Diagram. Generally R3 is less than 1Ω. C2 should generally be no smaller than 3.3 µF,
although that is dependent on the frequency and the allowable ripple amplitude at V
OUT1
. Experimentation is
usually necessary to determine the minimum value for C2, as the nature of the load may require a larger value. A
load which creates significant transients requires a larger value for C2 than a non-varying load.
D1: The important parameters are reverse recovery time and forward voltage. The reverse recovery time
determines how long the reverse current surge lasts each time the buck switch is turned on. The forward voltage
drop is significant in the event the output is short-circuited as it is mainly this diode’s voltage (plus the voltage
across the current limit sense resistor) which forces the inductor current to decrease during the off-time. For this
reason, a higher voltage is better, although that affects efficiency. A reverse recovery time of ≊30 ns, and a
forward voltage drop of ≊0.75V are preferred. The reverse leakage specification is important as that can
significantly affect efficiency. D1’s reverse voltage rating must be at least as great as the maximum V
IN
, and its
current rating must equal or exceed I
PK
Figure 8.
C1 and C5: C1’s purpose is to supply most of the switch current during the on-time, and limit the voltage ripple
at VIN, on the assumption that the voltage source feeding VIN has an output impedance greater than zero. If the
source’s dynamic impedance is high (effectively a current source), it supplies the average input current, but not
the ripple current.
At maximum load current, when the buck switch turns on, the current into VIN suddenly increases to the lower
peak of the inductor’s ripple current, ramps up to the peak value, then drop to zero at turn-off. The average
current during the on-time is the load current. For a worst case calculation, C1 must supply this average load
current during the maximum on-time. C1 is calculated from:
where
• Io is the load current
• t
ON
is the maximum on-time
• ΔV is the allowable ripple voltage at V
IN
(16)
C5’s purpose is to help avoid transients and ringing due to long lead inductance at V
IN
. A low ESR, 0.1 µF
ceramic chip capacitor is recommended, located close to the LM2695.
C4: The recommended value for C4 is 0.022 µF. A high quality ceramic capacitor with low ESR is recommended
as C4 supplies a surge current to charge the buck switch gate at turn-on. A low ESR also helps ensure a
complete recharge during each off-time.
C6: The capacitor at the SS pin determines the softstart time, i.e. the time for the reference voltage at the
regulation comparator, and the output voltage, to reach their final value. The time is determined from the
following:
(17)
PC BOARD LAYOUT
The LM2695 regulation, over-voltage, and current limit comparators are very fast, and respond to short duration
noise pulses. Layout considerations are therefore critical for optimum performance. The layout must be as neat
and compact as possible, and all of the components must be as close as possible to their associated pins. The
current loop formed by D1, L1, C2 and the SGND and ISEN pins should be as small as possible. The ground
connection from C2 to C1 should be as short and direct as possible.
12 Submit Documentation Feedback Copyright © 2006–2013, Texas Instruments Incorporated
Product Folder Links: LM2695