Datasheet
7
LT1611
Figure 8 shows the output voltage with a 50mA to 150mA
load step, using an AVX TAJ “B” case 22µF tantalum
capacitor at the output. Output perturbation is approxi-
mately 100mV as the load changes from 50mA to 150mA.
Steady-state ripple voltage is 20mV
P–P
, due to L1’s ripple
current and C3’s ESR. Step response can be improved by
adding a 3.3nF capacitor (C
PL
) as shown in Figure 9.
Settling time improves from 150µs to 40µs, although
steady-state ripple voltage does not improve. Figure 10
pictures the output voltage and switch pin voltage at
200ns 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
V
IN
V
IN
5V
–V
OUT
1611 F07
SW
L1A
22µH
L1B
22µH
D1
GND
LT1611
C1: AVX TAJB226M010
C2: TAIYO YUDEN LMK212BJ105MG
C3: SEE TEXT
D1: MBR0520
L1A, L1B: SUMIDA CLS62-220
C1
C3
C2
1µF
R2
10k
R1
C
PL
NFB
SHDN
+
+
R
L1
100Ω
R
L2
50Ω
Figure 7. Switching R
L2
Provides 50mA to 150mA
Load Step for LT1611 5V to –5V Converter
Figure 8. Load Step Response of LT1611
with 22µF Tantalum Output Capacitor
Figure 9. Addition of C
PL
to Figure 7’s Circuit
Improves Load Step Response. C
PL
= 3.3nF
Figure 10. 22µF “B” Case Tantalum Capacitor (AVX TAJ “B” Series)
Has ESR Resulting in 20mV
P–P
Voltage Ripple at Output
V
OUT
50mV/DIV
AC COUPLED
LOAD CURRENT
150mA
50mA
100µs/DIV
1611 F08
V
OUT
20mV/DIV
AC COUPLED
LOAD CURRENT
150mA
50mA
20µs/DIV
1611 F09
V
OUT
10mV/DIV
SWITCH VOLTAGE
5V/DIV
LOAD = 150mA 200ns/DIV
1611 F10