8
LT1614
Transient Response
The inverting architecture of the LT1614 can generate a
very low ripple output voltage. Recently available high
value ceramic capacitors can be used successfully in
LT1614 designs. The addition of a phase lead capacitor,
C
PL
, reduces output perturbations due to load steps when
lower value ceramic capacitors are used and connected in
parallel with feedback resistor R1. Figure 7 shows an
LT1614 inverting converter with resistor loads R
L1
and
R
L2
.
R
L1
is connected across the output, while R
L2
is
switched in externally via a pulse generator. Output volt-
age waveforms are pictured in subsequent figures, illus-
trating the performance of output capacitor type.
Figure 8 shows the output voltage with a 50mA to 200mA
load step, using an AVX TAJ “B” case 33µF tantalum
capacitor at the output. Output perturbation is approxi-
mately 250mV as the load changes from 50mA to 200mA.
Steady-state ripple voltage is 40mV
P–P
, due to L1’s ripple
current and C3’s ESR. Figure 9 pictures the output voltage
and switch pin voltage at 500ns per division. Note the
absence of high frequency spikes at the output. This is
easily repeatable with proper layout, described in the next
section.
OPERATIO
U
In Figure 10, output capacitor C3 is replaced by a ceramic
unit. These large value capacitors have ESR of 2mΩ or less
and result in very low output ripple. A 1nF capacitor, C
PL
,
connected across R1 reduces output perburbation due to
load step. This keeps the output voltage within 5% of
steady-state value. Figure 11 pictures the output and
switch nodes at 500ns per division. Output ripple is about
5mV
P-P
. Again, good layout is essential to achieve this low
noise performance.
Layout
The LT1614 switches current at high speed, mandating
careful attention to layout for best performance.
You will
not get advertised performance with careless layout.
Figure␣ 12
shows recommended component placement. Follow this
closely in your printed circuit layout. The cut ground
copper at D1’s cathode is essential to obtain the low noise
achieved in Figures 10 and 11’s oscillographs. Input
bypass capacitor C1 should be placed close to the LT1614
as shown. The load should connect directly to output
capacitor C2 for best load regulation. You can tie the local
ground into the system ground plane at C3’s ground
terminal.
COMPONENT SELECTION
Inductors
Each of the two inductors used with the LT1614 should
have a saturation current rating (where inductance is
approximately 70% of zero current inductance) of ap-
proximately 0.4A or greater. If the device is used in
“charge pump” mode, where there is only one inductor,
then its rating should be 0.75A or greater. DCR of the
inductors should be 0.4Ω or less. 22µH inductors are
called out in the applications schematics because these
Murata units are physically small and inexpensive. In-
creasing the inductance will lower ripple current, increas-
ing available output current. A coupled inductor of 33µH,
such as Coiltronics CTX33-2, will provide 290mA at –5V
from a 5V input. Inductance can be reduced if operating
from a supply voltage below 3V. Table 1 lists several
inductors that will work with the LT1614, although this is
not an exhaustive list. There are many magnetics vendors
whose components are suitable.
V
IN
V
IN
5V
–V
OUT
1614 F07
SW
L1
22µH
L2
22µH
D1
GND
LT1614
C1: AVX TAJB226M010
C2: TAIYO YUDEN LMK212BJ105MG
C3: AVX TAJB336M006 OR MURATA (SEE TEXT)
D1: MBR0520
L1, L2: MURATA LQH3C220
C1
C3
C2
1µF
R2
24.9k
R1
69.8k
C
PL
1nF
NFB
SHDN
+
+
R
L1
100Ω
R
L2
33Ω
R
C
C
C
V
C
Figure 7. Switching R
L2
Provides 50mA to 200mA
Load Step for LT1614 5V to –5V Converter
