LT1507 Linear Technology, LT1507 Datasheet - Page 16

• Manufacturer Part Number: LT1507
• Description: 500kHz Monolithic Buck Mode Switching Regulator
• Manufacturer: Linear Technology

APPLICATIONS
LT1507
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 V
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.
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
16
INPUT
Figure 6. Adding a Small PNP to Reduce Minimum
Start-Up Voltage
+
BAT85
V
D2
IN
GND
LT1507-3.3
BOOST
U
V
C
SENSE
0.22 F
SW
V
C
SW
C
C2
INFORMATION
to the boost capacitor. Note that
U
Q1
2N3906
BAT85
D3
W
D1
1N5818
L1
+
U
V
LT1507 • F06
OUT
C1
= 3.3V
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.
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
Figure 7. Adding External Slope Compensation for High
Sync Frequencies
C
1000pF
S
SYNC
5.2k
R
S
LT1507
GND
V
V
SW
C
R
470
C
2000pF
C
C
+
V
OUT
LT1507 • F07