Datasheet LT1614 (Analog Devices) - 8

LT1614
U OPERATIO Transient Response
In Figure 10, output capacitor C3 is replaced by a ceramic unit. These large value capacitors have ESR of 2mΩ or less The inverting architecture of the LT1614 can generate a and result in very low output ripple. A 1nF capacitor, C very low ripple output voltage. Recently available high PL, connected across R1 reduces output perburbation due to value ceramic capacitors can be used successfully in load step. This keeps the output voltage within 5% of LT1614 designs. The addition of a phase lead capacitor, steady-state value. Figure 11 pictures the output and CPL, reduces output perturbations due to load steps when switch nodes at 500ns per division. Output ripple is about lower value ceramic capacitors are used and connected in 5mV parallel with feedback resistor R1. Figure 7 shows an P-P. Again, good layout is essential to achieve this low noise performance. LT1614 inverting converter with resistor loads RL1 and RL2. RL1 is connected across the output, while RL2 is
Layout
switched in externally via a pulse generator. Output volt- age waveforms are pictured in subsequent figures, illus- The LT1614 switches current at high speed, mandating trating the performance of output capacitor type. careful attention to layout for best performance. You will not get advertised performance with careless layout. Figure␣ 12 Figure 8 shows the output voltage with a 50mA to 200mA shows recommended component placement. Follow this load step, using an AVX TAJ “B” case 33µF tantalum closely in your printed circuit layout. The cut ground capacitor at the output. Output perturbation is approxi- copper at D1’s cathode is essential to obtain the low noise mately 250mV as the load changes from 50mA to 200mA. achieved in Figures 10 and 11’s oscillographs. Input Steady-state ripple voltage is 40mVP–P, due to L1’s ripple bypass capacitor C1 should be placed close to the LT1614 current and C3’s ESR. Figure 9 pictures the output voltage as shown. The load should connect directly to output and switch pin voltage at 500ns per division. Note the capacitor C2 for best load regulation. You can tie the local absence of high frequency spikes at the output. This is ground into the system ground plane at C3’s ground easily repeatable with proper layout, described in the next terminal. section.
COMPONENT SELECTION
C2 L1 L2
Inductors
1µF 22µH 22µH VIN 5V Each of the two inductors used with the LT1614 should D1 have a saturation current rating (where inductance is V –V IN SW OUT approximately 70% of zero current inductance) of ap- SHDN R R1 CPL + L2 LT1614 33Ω 69.8k 1nF R proximately 0.4A or greater. If the device is used in C1 L1 V NFB 100Ω C C3 “charge pump” mode, where there is only one inductor, + GND R2 RC 24.9k then its rating should be 0.75A or greater. DCR of the CC inductors should be 0.4Ω or less. 22µH inductors are C1: AVX TAJB226M010 called out in the applications schematics because these 1614 F07 C2: TAIYO YUDEN LMK212BJ105MG Murata units are physically small and inexpensive. In- C3: AVX TAJB336M006 OR MURATA (SEE TEXT) D1: MBR0520 creasing the inductance will lower ripple current, increas- L1, L2: MURATA LQH3C220 ing available output current. A coupled inductor of 33µH,
Figure 7. Switching RL2 Provides 50mA to 200mA
such as Coiltronics CTX33-2, will provide 290mA at – 5V
Load Step for LT1614 5V to – 5V Converter
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. 8