LTC3770_15 Datasheet, PDF (15/24 Pages) Linear Technology – Fast No RSENSE Step-Down Synchronous Controller with Margining, Tracking and PLL

LTC3770
APPLICATIONS INFORMATION
Top MOSFET Driver Supply (C_B, D_B)
An external bootstrap capacitor C_B connected to the BOOST
pin supplies the gate drive voltage for the topside MOSFET.
This capacitor is charged through diode D_B from INTV_CC
when the switch node is low. When the top MOSFET turns
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 depends 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.
V_IN
TG
LTC3770
R4
SW
FCB
R3
SGND
BG
PGND
+ ^VIN
C_IN
1N4148
•
+
T1 ^• ₊
1:N
V_OUT2
C_OUT2
1μF
V_OUT1
C_OUT
3770 F05
Figure 5. Secondary Output Loop
In addition to providing a logic input to forced 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
normally set as shown in Figure 5 by the turns ratio N
of the transformer. However, if the controller goes into
discontinuous 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+
R4
R3
⎞
⎠⎟
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
ΔI_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
ambient 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
maximum lies above it. Consult the MOSFET manufacturer
for further guidelines.
3770fc
15