LT1956IGN datasheet(25/28 Pages) LINER

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LT1956/LT1956-5

1956f

current in L2 and C4 flows via the catch diode D3, charging

the negative output capacitor C6. If the negative output is

not loaded enough, it can go severely unregulated (be-

come more negative). Figure 14b shows the maximum

allowable –5V output load current (vs load current on the

5V output) that will maintain the –5V output within 3%

tolerance. Figure 14c shows the –5V output voltage regu-

lation vs its own load current when plotted for three

separate load currents on the 5V output. The efficiency of

the dual output converter circuit shown in Figure 14a is

given in Figure 14d.

POSITIVE-TO-NEGATIVE CONVERTER

The circuit in Figure 15 is a positive-to-negative topology

using a grounded inductor. It differs from the standard

approach in the way the IC chip derives its feedback signal

because the LT1956 accepts only positive feedback sig-

nals. The ground pin must be tied to the regulated negative

output. A resistor divider to the FB pin, then provides the

proper feedback voltage for the chip.

The following equation can be used to calculate maximum

load current for the positive-to-negative converter:

APPLICATIO S I FOR ATIO

f L

MAX

OUT













–

()(

)

()( )( )

()(

– . )

(– . )(

)

IP = maximum rated switch current

VIN = minimum input voltage

VOUT = output voltage

VF = catch diode forward voltage

0.3 = switch voltage drop at 1.5A

Example: with VIN(MIN) = 5.5V, VOUT = 12V, L = 15µH,

VF = 0.63V, IP = 1.5A: IMAX = 0.36A.

INDUCTOR VALUE

The criteria for choosing the inductor is typically based on

ensuring that peak switch current rating is not exceeded.

This gives the lowest value of inductance that can be used,

but in some cases (lower output load currents) it may give

a value that creates unnecessarily high output ripple

voltage.

The difficulty in calculating the minimum inductor size

needed is that you must first decide whether the switcher

will be in continuous or discontinuous mode at the critical

point where switch current reaches 1.5A. The first step is

to use the following formula to calculate the load current

above which the switcher must use continuous mode. If

your load current is less than this, use the discontinuous

mode formula to calculate minimum inductor needed. If

load current is higher, use the continuous mode formula.

Output current where continuous mode is needed:

CONT

OUT

() ( )

()(

)

Minimum inductor discontinuous mode:

MIN

OUT

()(

)

()( )

OUTPUT**

–12V, 0.25A

VIN

12V

1956 F15

0.1

µF

10MQO60N

36.5k

100

µF

20V TANT

2.2

µF

25V

MMSD914TI

L1*

µH

BOOST

LT1956

VIN

GND

4.12k

* INCREASE L1 TO 10

µH OR 18µH FOR HIGHER CURRENT APPLICATIONS.

SEE APPLICATIONS INFORMATION

** MAXIMUM LOAD CURRENT DEPENDS ON MINIMUM INPUT VOLTAGE

AND INDUCTOR SIZE. SEE APPLICATIONS INFORMATION

Figure 15. Positive-to-Negative Converter

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