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LT1507

APPLICATIONS INFORMATION

The circuit in Figure 6 will allow operation at light loads

with low input voltages. It uses a small PNP to charge the

boost capacitor (C2) and an extra diode (D3) to complete

the power path from VSW to the boost capacitor. Note that

the diodes have been changed to Schottky BAT85s to

optimize low voltage operation. Figure 5 shows that with

the added PNP, minimum load current can be reduced to

6mA and still guarantee proper start-up with 4.7V input.

problems. For low input voltage, high sync frequency

applications, the circuit shown in Figure 7 can be used to

generate an external slope compensation ramp that elimi-

nates subharmonic oscillation. See Frequency Compen-

sation section for a discussion of an entirely different

cause of subharmonic switching before assuming that the

cause is insufficient slope compensation. Application Note

19 has more details on the theory of slope compensation.

VIN

VSW

SENSE

BOOST

GND

LT1507-3.3

BAT85

0.22

µF

1N5818

INPUT

LT1507 • F06

BAT85

2N3906

VOUT = 3.3V

Figure 6. Adding a Small PNP to Reduce Minimum

Start-Up Voltage

SYNCHRONIZING

The LT1507 SYNC pin is used to synchronize the internal

oscillator to an external signal. It is directly logic compat-

ible and can be driven with any signal between 10% and

90% duty cycle. The synchronizing range is equal to

initial

operating frequency up to 1MHz (above 700kHz external

slope compensation may be needed). This means that

minimum practical sync frequency is equal to the worst-

case

high self-oscillating frequency (560kHz) not the

typical operating frequency of 500kHz. Caution should be

used when synchronizing above 700kHz because at higher

sync frequencies, the amplitude of the internal slope

compensation used to prevent subharmonic switching is

reduced. This type of subharmonic switching only occurs

at input voltages less than twice the output voltage and

shows up as alternating pulse widths at the switch node.

It does not cause the regulator to lose regulation, but

switch frequency content down to 100kHz may be objec-

tionable. Higher inductor values will tend to eliminate

SYNC

VSW

GND

LT1507

VOUT

LT1507 • F07

470

Ω

5.2k

1000pF

2000pF

Figure 7. Adding External Slope Compensation for High

Sync Frequencies

External Slope Compensation Ramp

The LT1507 is a current mode switching regulator and

therefore, it requires something called “slope compensa-

tion”

when operated above 50% duty cycle in continuous

mode. This condition occurs when input voltage is less

than twice output voltage. Slope compensation adds a

ramp to the switch current sense signal generated on the

chip during switch ON time. Typically the ramp is gener-

ated from a portion of the internal oscillator waveform. In

the LT1507, the ramp is arranged to be zero until the

oscillator waveform reaches about 40% of its final value.

This minimizes the total amount of ramp added to switch

current. The reason for doing it this way is that the ramp

subtracts from switch current limit, so that switch current

limit would be considerably lower at high duty cycle

compared to low duty cycle if the ramp existed at all duty

cycles. By starting the ramp at the 40% point, changes in

current limit are minimized. No ramp is needed when

operating below 50% duty cycle.

Problems can occur with this technique if the regulator is

used with a combination of high external sync frequency

and more than 50% duty cycle. The basic sync function

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