Datasheet

C6 =
2.5V
t
SS
x 12 PA
ESR
(min)
=
33 mA
50 mV
= 1.5:
I
OR(min)
=
L1
max
x F
S(max)
x V
IN(min)
V
OUT
x (V
IN(min)
- V
OUT
)
180 PH x 315 kHz x 8V
5V x (8V - 5V)
= 33 mAp-p
=
LM2694
www.ti.com SNVS444A –MAY 2006–REVISED APRIL 2013
Normally a lower value can be used for C1 since the above calculation is a worst case calculation which
assumes the power source has a high source impedance. A quality ceramic capacitor with a low ESR should be
used for C1.
C2 and R3: Since the LM2694 requires a minimum of 25 mVp-p of ripple at the FB pin for proper operation, the
required ripple at V
OUT
is increased by R1 and R2, and is equal to:
V
RIPPLE
= 25 mVp-p x (R1 + R2)/R2 = 50 mVp-p (18)
This necessary ripple voltage is created by the inductor ripple current acting on C2’s ESR + R3. First, the
minimum ripple current, which occurs at minimum VIN, maximum inductor value, and maximum frequency, is
determined.
(19)
The minimum ESR for C2 is then equal to:
(20)
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. The value chosen for C2 is application dependent, and it is recommended that it be
no smaller than 3.3 µF. C2 affects the ripple at V
OUT
, and transient response. Experimentation is usually
necessary to determine the optimum value for C2.
C3: The capacitor at the VCC pin provides noise filtering and stability, prevents false triggering of the V
CC
UVLO
at the buck switch on/off transitions, and limits the peak voltage at V
CC
when a high voltage with a short rise time
is initially applied at V
IN
. C3 should be no smaller than 0.1 µF, and should be a good quality, low ESR, ceramic
capacitor, physically close to the IC pins.
C4: The recommended value for C4 is 0.022 µF. A high quality ceramic capacitor with low ESR is recommended
as C4 supplies the surge current to charge the buck switch gate at each turn-on. A low ESR also ensures a
complete recharge during each off-time.
C5: This capacitor suppresses transients and ringing due to lead inductance at VIN. A low ESR, 0.1 µF ceramic
chip capacitor is recommended, located physically close to the LM2694.
C6: The capacitor at the SS pin determines the soft-start time, i.e. the time for the reference voltage at the
regulation comparator, and the output voltage, to reach their final value. The capacitor value is determined from
the following:
(21)
For a 5 ms softstart time, C6 calculates to 0.024 µF.
D1: A Schottky diode is recommended. Ultra-fast recovery diodes are not recommended as the high speed
transitions at the SW pin may inadvertently affect the IC’s operation through external or internal EMI. The diode
should be rated for the maximum V
IN
(30V), the maximum load current (0.6A), and the peak current which occurs
when current limit and maximum ripple current are reached simultaneously (I
PK
in Figure 12), previously
calculated to be 0.92A. The diode’s forward voltage drop affects efficiency due to the power dissipated during the
off-time. The average power dissipation in D1 is calculated from:
P
D1
= V
F
x I
O
x (1 - D)
where
• I
O
is the load current, and D is the duty cycle (22)
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