LTC3770 Datasheet, PDF (15/24 Pages) Linear Technology – Synchronous Controller with Margining, Tracking and PLL

LTC3770
APPLICATIO S I FOR ATIO
on, the switch node rises to V_IN and the BOOST pin rises
to approximately V_IN + INTV_CC. The boost capacitor needs
to store about 100 times the gate charge required by the
top MOSFET. In most applications 0.1µF to 0.47µF, X5R or
X7R dielectric capacitor is adequate.
Discontinuous 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 0.6V threshold enables discontinuous
operation where the bottom MOSFET turns off when
inductor current reverses. The load current at which
current reverses and discontinuous operation begins de-
pends on the amplitude of the inductor ripple current and
will vary with changes in V_IN. Tying the FCB pin below the
0.6V 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 TRACK/SS voltage
is 20% below the reference voltage during soft-start or
tracking up. Forced continuous mode of operation is also
disabled when the TRACK/SS voltage is below 0.1V during
tracking down operation. 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
flyback winding output when the primary is operating in
discontinuous mode. The secondary output V_OUT2 is nor-
mally set as shown in Figure 5 by the turns ratio N of the
V_IN
TG
LTC3770
SW
R4
FCB
R3
SGND
BG
PGND
+ ^VIN
C_IN
1N4148
•
+
V_OUT2
C_OUT2
1µF
T1 ^• ₊
V_OUT1
1:N
C_OUT
3770 F05
Figure 5. Secondary Output Loop
transformer. However, if the controller goes into discon-
tinuous 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) = 0.6V^{⎛⎝⎜}1+ ^{RR34⎞⎠⎟}
Fault Conditions: Current Limit and Foldback
The maximum inductor current is inherently limited in a
current mode controller by the maximum sense voltage. In
the LTC3770, the maximum sense voltage is controlled by
the voltage on the V_RNG pin. With valley current control,
the maximum sense voltage and the sense resistance
determine the maximum allowed inductor valley current.
The corresponding output current limit is:
I_LIMIT
=
V_SNS(MAX)
R_DS(ON) ρ_T
+
1
2
∆I_L
The current limit value should be checked to ensure that
I_LIMIT(MIN) > I_OUT(MAX). The minimum value of current limit
generally occurs with the largest V_IN at the highest ambi-
ent temperature, conditions that cause the largest power
loss in the converter. Note that it is important to check for
self-consistency between the assumed MOSFET junction
temperature and the resulting value of I_LIMIT which heats
the MOSFET switches.
Caution should be used when setting the current limit
based upon the R_DS(ON) of the MOSFETs. The maximum
current limit is determined by the minimum MOSFET on-
resistance. Data sheets typically specify nominal and
maximum values for R_DS(ON), but not a minimum. A
reasonable assumption is that the minimum R_DS(ON) lies
the same percentage below the typical value as the maxi-
mum lies above it. Consult the MOSFET manufacturer for
further guidelines.
To further limit current in the event of a short circuit to
ground, the LTC3770 includes foldback current limiting. If
the output falls by more than 60%, then the maximum
sense voltage is progressively lowered to about one tenth
of its full value.
3770f
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