Datasheet

LTC3812-5
25
38125fc
where H(s) was given in equation 2 and A(s) depends on
compensation circuit used:
Type 2:
A (s)=
1+ s•R2•C1
s•R1• C1+C2
()
•1+ s•R2•
C1• C2
C1+C2
⎛
⎝
⎜
⎞
⎠
⎟
Type 3:
A (s)=
1
s•R1• C1+C2
()
•
1+s• R1+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-SKIPPING MODE OPERATION AND FCB PIN
The FCB pin determines whether the bottom MOSFET
remains on when current reverses in the inductor. Tying
this pin above its 0.8V threshold enables pulse-skipping
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 FCB pin below the 0.8V
threshold forces continuous synchronous operation,
allowing current to reverse at light loads and maintaining
high frequency operation. To prevent forcing current back
into the main power supply, potentially boosting the input
supply to a dangerous voltage level, forced continuous
mode of operation is disabled when the RUN/SS voltage
is below 2.5V during soft-start or tracking. During these
two periods, the PGOOD signal is forced low.
In addition to providing a logic input to force continuous
operation, the FCB pin provides a mean to maintain a
fl yback winding output when the primary is operating
in pulse-skipping mode. The secondary output V
OUT2
is
normally set as shown in Figure 13 by the turns ratio N
of the transformer. However, if the controller goes into
pulse-skipping mode and halts switching due to a light
primary load current, then V
OUT2
will droop. An external
resistor divider from V
OUT2
to the FCB pin sets a minimum
voltage V
OUT2(MIN)
below which continuous operation is
forced until V
OUT2
has risen above its minimum.
V
OUT2(MIN)
= 0.8V 1+
R4
R3






Table 1
FCB PIN CONDITION
DC Voltage: 0V to 0.75V Forced Continuous
Current Reversal Enabled
DC Voltage: ≥0.85V Pulse-Skipping Mode Operation
No Current Reversal
Feedback Resistors Regulating a Secondary Winding
APPLICATIONS INFORMATION
Figure 13. Secondary Output Loop
V
IN
LTC3812-5
SGND
FCB
TG
SW
R3
R4
38125 F13
T1
1:N
BG
PGND
+
C
OUT2
1μF
V
OUT1
V
OUT2
V
IN
+
C
IN
1N4148
•
•
+
C
OUT