LTC3727LX-1_15 Datasheet, PDF (26/28 Pages) Linear Technology – High Efficiency, 2-Phase Synchronous Step-Down Switching Regulator

LTC3727LX-1
APPLICATIO S I FOR ATIO
sensing inputs or inadequate loop compensation. Over-
compensation of the loop can be used to tame a poor PC
layout if regulator bandwidth optimization is not required.
Only after each controller is checked for its individual
performance should both controllers be turned on at the
same time. A particularly difficult region of operation is
when one controller channel is nearing its current com-
parator trip point when the other channel is turning on its
top MOSFET. This occurs around 50% duty cycle on either
channel due to the phasing of the internal clocks and may
cause minor duty cycle jitter.
Reduce V_IN from its nominal level to verify operation of the
regulator in dropout. Check the operation of the under-
voltage lockout circuit by further lowering V_IN while moni-
toring the outputs to verify operation.
Investigate whether any problems exist only at higher
output currents or only at higher input voltages. If prob-
lems coincide with high input voltages and low output
currents, look for capacitive coupling between the BOOST,
SW, TG, and possibly BG connections and the sensitive
voltage and current pins. The capacitor placed across the
current sensing pins needs to be placed immediately
adjacent to the pins of the IC. This capacitor helps to
minimize the effects of differential noise injection due to
high frequency capacitive coupling. If problems are en-
countered with high current output loading at lower input
voltages, look for inductive coupling between C_IN, Schottky
and the top MOSFET components to the sensitive current
and voltage sensing traces. In addition, investigate com-
mon ground path voltage pickup between these compo-
nents and the SGND pin of the IC.
An embarrassing problem, which can be missed in an
otherwise properly working switching regulator, results
when the current sensing leads are hooked up backwards.
The output voltage under this improper hookup will still be
maintained but the advantages of current mode control
will not be realized. Compensation of the voltage loop will
be much more sensitive to component selection. This
behavior can be investigated by temporarily shorting out
the current sensing resistor—don’t worry, the regulator
will still maintain control of the output voltage.
26
0.1µF
27pF
20k
1%
105k
1%
1000pF
0.01µF
10k
1000pF
33pF
f_SYNC
15k _220pF
33pF
3.3V
15k _220pF
20k
1%
280k
27pF
1%
1000pF
1
RUN/SS1
28
PGOOD
2 _SENSE1+
27
TG1
3 _SENSE1–
26
SW1
4
V_OSENSE1
5
PLLFLTR
6
PLLIN
25
BOOST1
24
V_IN
23
BG1
7
FCB
22
EXTV_CC
8
LTC3727LX-1
21
I_TH1
INTV_CC
9
SGND
20
PGND
10
3.3V_OUT
11
I_TH2
12
V_OSENSE2
13 _SENSE2–
19
BG2
18
BOOST2
17
SW2
16
TG2
14 _SENSE2+
15
RUN/SS2
V_PULL-UP
(<7V)
PGOOD
L1
8µH
0.015Ω
0.1µF
CMDSH-3
1µF
10V
4.7µF
CMDSH-3
0.1µF
M1A
10Ω
0.1µF
M2A
M1B
22µF
50V
D1
MBRM
140T3
C_OUT1
47µF
6.3V
C_OUT2 100µF 16V
D2
MBRM
140T3
M2B
L2
15µH
0.015Ω
V_OUT1
5V
5A; 6A PEAK
GND
V_IN
15V TO
28V
V_OUT2
12V
4A; 5A PEAK
0.1µF
C_OUT1: PANASONIC EEFCDOJ470R
C_OUT2: SANYO OS-CON 16SVP100M
V_IN: 15V TO 28V
V_OUT: 5V, 5A/12V, 4A
SWITCHING FREQUENCY = 250kHz
MI, M2: FAIRCHILD FDS6680A
3727LX1 F12
L1: 8µH SUMIDA CDEP134-8R0
L2: 15µH COILTRONICS UP4B-150
Figure 12. LTC3727LX-1 12V/4A, 5V/5A Regulator with External Frequency Synchronization
3727lx1fa