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LT1614

Inverting 600kHz Switching Regulator

Linear Technology 

LT1614

U

OPERATIO

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,

CPL, 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 RL1 and

RL2. RL1 is connected across the output, while RL2 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 40mVP–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.

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, CPL,

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

5mVP-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

L1

VIN

22µH

5V

+

C1

RC

VIN

SW

SHDN

LT1614

VC

NFB

GND

CC

C2

1µF

L2

22µH

R1

69.8k

R2

24.9k

D1

CPL

1nF

C3

–VOUT

RL2

RL1 33Ω

100Ω

C1: AVX TAJB226M010

C2: TAIYO YUDEN LMK212BJ105MG

C3: AVX TAJB336M006 OR MURATA (SEE TEXT)

D1: MBR0520

L1, L2: MURATA LQH3C220

1614 F07

Figure 7. Switching RL2 Provides 50mA to 200mA

Load Step for LT1614 5V to – 5V Converter

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.

8

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