Datasheet

SW
FB
V
OUT
R2
LM2694
R1
C2
L1
CB
CARA
SW
FB
LM2694
V
OUT
Cff
R2
L1
R1
C2
R3
FB
SW
L1
C2
R3
LM2694
R1
R2
V
OUT
LM2694
www.ti.com SNVS444A –MAY 2006–REVISED APRIL 2013
MINIMUM LOAD CURRENT
The LM2694 requires a minimum load current of 500 µA. If the load current falls below that level, the bootstrap
capacitor (C4) may discharge during the long off-time, and the circuit will either shutdown, or cycle on and off at
a low frequency. If the load current is expected to drop below 500 µA in the application, R1 and R2 should be
chosen low enough in value so they provide the minimum required current at nominal V
OUT
.
LOW OUTPUT RIPPLE CONFIGURATIONS
For applications where low output voltage ripple is required the output can be taken directly from the low ESR
output capacitor (C2) as shown in Figure 17. However, R3 slightly degrades the load regulation. The specific
component values, and the application determine if this is suitable.
Figure 17. Low Ripple Output
Where the circuit of Figure 17 is not suitable for reducing output ripple, the circuits of Figure 18 or Figure 19 can
be used.
Figure 18. Low Output Ripple Using a Feedforward Capacitor
In Figure 18, Cff is added across R1 to AC-couple the ripple at V
OUT
directly to the FB pin. This allows the ripple
at V
OUT
to be reduced, in some cases considerably, by reducing R3. In the circuit of Figure 14, the ripple at V
OUT
ranged from 50 mVp-p at V
IN
= 8V to 100 mVp-p at V
IN
= 30V. By adding a 2700 pF capacitor at Cff and
reducing R3 to 0.75Ω, the V
OUT
ripple is reduced by 50%.
Figure 19. Minimum Output Ripple Using Ripple Injection
To reduce V
OUT
ripple further, the circuit of Figure 19 can be used. R3 has been removed, and the output ripple
amplitude is determined by C2’s ESR and the inductor ripple current. RA and CA are chosen to generate a 40-50
mVp-p sawtooth at their junction, and that voltage is AC-coupled to the FB pin via CB. In selecting RA and CA,
V
OUT
is considered a virtual ground as the SW pin switches between V
IN
and -1V. Since the on-time at SW varies
inversely with V
IN
, the waveform amplitude at the RA/CA junction is relatively constant. Typical values for the
additional components are RA = 110k, CA = 2700 pF, and CB = 0.01 µF.
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