12
LT1619
1619fa
Increasing Ramp Compensation While Synchronizing
The LT1619 is synchronized by forced discharge of the
internal timing ramp. The timing ramp amplitude de-
creases as the synchronization frequency increases. Since
the internal compensation ramp is derived from the timing
ramp, reduced timing ramp results in diminished com-
pensating ramp. If the LT1619 is synchronized at frequen-
cies 20% to 30% higher than the free-running frequency,
external ramp compensation will be required. Figures 12
and 13 show two such schemes.
In both figures the compensating ramps are kept linear by
making R11-C1 and R14-C2 products substantially higher
than the synchronizing period. The compensation ramps,
whose peak amplitudes are made between 1/4 to 1/3 of the
current limit threshold, are developed across R13. As a
result, the effective current limit threshold is reduced by
the sum of the compensating ramp and the offset voltage
developed across R13 due to the SENSE pin input bias
current (see Figure 5). Moreover, the current limit thresh-
old becomes duty cycle dependent.
PC Board Layout and Other Practical Considerations
The following is recommended for PC board layout:
1. Trace lengths of the branches carrying switched cur-
rent should be kept short. For example, in the boost
converter of Figure 1, the circuit loop formed by M1,
R
SENSE
, D1 and C
OUT
carries switched current. The size
of this loop must be minimized. R
SENSE
and C
OUT
should be grounded to a single point on a large ground
plane. This reduces switching noise and overall con-
verter jitter. It is also preferable to ground the input
capacitor C1 close to the common point between C
OUT
and R
SENSE
although this is less important.
2. Keep the trace between the sense resistor and the
SENSE pin short. When sensing high switch current,
Kelvin connection to R
SENSE
is necessary.
3. Bypass both the V
IN
and DRV pins with ceramic capaci-
tors next to the IC and the ground plane.
4. Keep high voltage switching nodes, such as the drain
and gate of the MOSFET, away from the FB and V
C
pins.
5. Use inductor so that its ripple current is between 1/4
and 1/3 of its peak current. Steeper inductor current
ramp results in sharper PWM comparator switching,
hence less jitter.
6. In most cases, filtering the current sense signal is not
necessary for jitter-free operation.
Figure 14 is the PC board layout for the 5V/8A and 12V/5A
boost converters shown in Figures 15a and 16a.
S/S
FB
V
C
GND
8
7
6
5
1
2
3
4
V
IN
DRV
GATE
SENSE
LT1619
R
SENSE
CLK
D2
1N4148
Q1
2N2222
R11
100k
R12
2200Ω
R13
51Ω
MAIN POWER
TRANSISTOR
C1
220pF
1619 F12
S/S
FB
V
C
GND
8
7
6
5
1
2
3
4
V
IN
DRV
GATE
SENSE
LT1619
R
SENSE
CLK
D2
1N4148
D3
1N4148
R14
8200Ω
R15
2400Ω
R13
51Ω
C2
2.2nF
1619 F13
Figure 12. Increasing Ramp Compensation. Q1 Buffers the C1
Ramp. D2 Discharges C1. Values Shown are for 10V Gate Drive
and 15mV Ramp Across R13 at 90% Duty Cycle and 500kHz
Figure 13. Externally Increasing Ramp Compensation. Similar
to Figure 12 Except That C2 is Not Buffered with Transistor
APPLICATIO S I FOR ATIO
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