LT1507
U
W U U
APPLICATIONS INFORMATION
works by prematurely tripping the oscillator before it
reaches its normal peak value. For instance, if the oscilla-
tor is synchronized at twice its nominal frequency, oscil-
lator amplitude will drop by half. A ramp which previously
started at the 40% point now starts at the 80% point! This
effectively blocks slope compensation and the regulator
may respond with fluctuating pulse widths, a “phase
oscillation” if you will. The regulator output stays in
regulation but subharmonic frequencies are generated at
the switch node.
ForVIN =4.7, VOUT =3.3V, f=1MHz, L=5µHandDCS =25%:
(6.6 − 4.7)(1− 0.25)
V
≥
= 71mV
P-P
6
−6
2 1 10
5 10
1.8
ToavoidsmallvaluesofRS, thecompensationcapacitor(CC)
should be made as small as possible. 2000pF will work in
most situations. If we increase VPP to 90mV for a little
cushion, RS will be:
The solution to this problem is to generate an external
ramp that replaces the missing internal ramp. As it turns
out, this is not difficult if the sync signal can be arranged
tohaveafairlylowdutycycle(<35%). Therampiscreated
by AC coupling a resistor from the sync signal to the
compensation capacitor as shown in Figure 7. This gener-
ates a negative ramp on the VC pin during switch ON time
that emulates the missing internally generated ramp.
Amplitude of the ramp should be about 100mV to 200mV
peak-to-peak. The formulas for calculating the values of
RS and CS are shown below. Note that the CS value is
unimportant as long as it exceeds the value given. The
formula assures that the impedance of CS will be small
compared to RS.
(5)(0.25)(0.75)
R =
= 5.2k
S
−9
6
0.09 2 10
1 10
(
)
(
)
20
C ≥
= 612pF
6
2π 1 10
5200
(
)
(
)
THERMAL CALCULATIONS
Power dissipation in the LT1507 chip comes from four
sources: switch DC loss, switch AC loss, boost circuit
current and input quiescent current. The formulas below
show how to calculate each of these losses. These formu-
las assume continuous mode operation, so they should
notbeusedforcalculatingefficiencyatlightloadcurrents.
V
(DC )(1− DC )
S S
SYNC
R =
S
Switch loss:
V
(C )(f)
C
P-P
2
20
2π(f)(R )
R
(I
) (V
V
IN
)
SW OUT
OUT
C >
S
P
=
+16ns(I )(V )(f)
OUT IN
SW
S
VSYNC = Peak-to-peak value of sync signal
DCS = Duty cycle of incoming sync signal
VP-P = Desired amplitude of ramp
f = Sync frequency
Boost current loss:
2
V
I
OUT
75
OUT
P
=
0.008 +
BOOST
V
IN
Theoretical minimum amplitude for the ramp, assuming
no internal ramp, is:
Quiescent current loss:
P = V (0.003)+ V (0.005)
Q
IN
OUT
(2V
− V )(1−DC )
IN S
OUT
V
≥
P-P
RSW = Switch resistance (≈ 0.4Ω)
16ns = Equivalent switch current/voltage overlap time
f = Switching frequency
2(f)(L)(g
)
mP
gmP = Transconductance from VC pin to switch current
(1.8A/V for the LT1507).
17
Linear [ Linear ]
