LTC3810-5 Datasheet, PDF (26/36 Pages) Linear Technology – 60V Current Mode Synchronous Switching Regulator Controller

LTC3810-5
APPLICATIONS INFORMATION
TYPE 3 Loop:
K
=
tan²


BOOST
4
+
45°^
C2
=
2
•
f
1
•G
•
R_FB1
C1= C2(K
1)
R2
=
2
•
K
f•
C1
R3
=
R_FB1
K
1
C3 = _2f
1
K • R3
R_B
=
V_REF (R_FB1)
V_OUT
V_REF
SPICE or mathematical software can be used to generate
the gain/phase plots for the compensated power supply to
do a sanity check on the component values before trying
them out on the actual hardware. For software, use the
following transfer function:
T(s) = A(s)H(s)
where H(s) was given in Equation 2 and A(s) depends on
compensation circuit used:
Type 2:
A
(s) =
s
•
R_FB1
•
(C2
1+ s
+ C3)
• R3 •
•



1+
C2
s • R3
•
C2 • C3 
C2 + C3
Type 3:
A
(s) =
s
•
R_FB1
1
• (C2
+
C3)
•
( ) ^{1+ s •} (^R_FB1 ^{+ R3}) ^{• C3 •} (^{1+ s •R2 • C1})
(1+
s
•
R3
•
C3)
•



1+
s
•
R2
•
C1• C2
C1+ C2


For SPICE, replace VSTIM line in the previous PSPICE
code with following code and generate a gain/phase plot
of V(out)/V(outin):
rfb1 outin vfb 52.5k
rfb2 vfb 0 10k
eithx ithx 0 laplace {0.8-v(vfb)} =
{1/(1+s/1000)}
eith ith 0 value={limit(1e6*v(ithx),0,2.4)}
cc1 ith vfb 4p
cc2 ith x1 8p
rc x1 vfb 210k
rf outin x2 11k ;delete this line for Type 2
cf x2 vfb 120p ;delete this line for Type 2
vstim out outin dc=0 ac=1m
Pulse Skip Mode Operation and MODE/SYNC Pin
The MODE/SYNC pin determines whether the bottom
MOSFET remains on when current reverses in the inductor.
Tying this pin above its 0.8V threshold enables pulse skip
mode operation where the bottom MOSFET turns off when
inductor current reverses. The load current at which current
reverses and discontinuous operation begins depends on
the amplitude of the inductor ripple current and will vary
with changes in V_IN. Tying the MODE/SYNC pin below the
0.8V threshold forces continuous synchronous operation,
allowing current to reverse at light loads and maintain-
ing high frequency operation. To prevent forcing current
back into the main power supply, potentially boosting the
input supply to a dangerous voltage level, forced continu-
ous mode of operation is disabled when the TRACK/SS
voltage is below the reference voltage during soft-start
or tracking. During these two periods, the PGOOD signal
is forced low.
Table 1
MODE/SYNC PIN
DC Voltage: 0V to 0.75V
DC Voltage: ≥ 0.85V
Feedback Resistors
Ext. Clock 0V to ≥ 2V
CONDITION
Forced Continuous
Current Reversal Enabled
Pulse Skip Mode Operation
No Current Reversal
Regulating a Secondary Winding
Forced Continuous
Current Reversal Enabled
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