LTC3703 Linear Technology, LTC3703 Datasheet - Page 18

• Manufacturer Part Number: LTC3703
• Description: 100V Synchronous Switching Regulator Controller
• Manufacturer: Linear Technology

LTC3703
APPLICATIO S I FOR ATIO
Bottom MOSFET Source Supply (BGRTN)
The bottom gate driver, BG, switches from DRV
where BGRTN can be a voltage between ground and –5V.
Why not just keep it simple and always connect BGRTN to
ground? In high voltage switching converters, the switch
node dV/dt can be many volts/ns, which will pull up on the
gate of the bottom MOSFET through its Miller capacitance.
If this Miller current, times the internal gate resistance of
the MOSFET plus the driver resistance, exceeds the thresh-
old of the FET, shoot-through will occur. By using a nega-
tive supply on BGRTN, the BG can be pulled below ground
when turning the bottom MOSFET off. This provides a few
extra volts of margin before the gate reaches the turn-on
threshold of the MOSFET. Be aware that the maximum
voltage difference between DRV
for example, V
DRV
Current Limit Programming
Programming current limit on the LTC3703 is straight
forward. The I
maximum allowable voltage drop across the bottom
MOSFET. The voltage across the MOSFET is set by its on-
resistance and the current flowing in the inductor, which
is the same as the output current. The LTC3703 current
limit circuit inverts the negative voltage across the MOSFET
before comparing it to the voltage at I
current limit to be set with a positive voltage.
To set the current limit, calculate the expected voltage
drop across the bottom MOSFET at the maximum desired
current and maximum junction temperature:
where
V
internal 12 A pull-up and an external resistor:
The current limit value should be checked to ensure that
I
generally occurs with the largest V
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
18
LIMIT(MIN)
PROG
V
R
PROG
IMAX
CC
is then programmed at the I
pin is now 13V instead of 15V.
is explained in the MOSFET Selection section.
= V
= (I
> I
PROG
OUT(MAX)
LIMIT
MAX
BGRTN
/12 A
)(R
pin sets the current limit by setting the
U
DS(ON)
. The minimum value of current limit
= –2V, the maximum voltage on
U
)(1 + )
CC
IN
and BGRTN is 15V. If,
W
at the highest ambi-
MAX
MAX
pin using the
, allowing the
CC
U
to BGRTN
temperature and the resulting value of I
the MOSFET switches.
Caution should be used when setting the current limit
based upon the R
current limit is determined by the minimum MOSFET on-
resistance. Data sheets typically specify nominal and
maximum values for R
reasonable assumption is that the minimum R
the same amount below the typical value as the maximum
lies above it. Consult the MOSFET manufacturer for further
guidelines.
For best results, use a V
500mV. Values outside of this range may give less accu-
rate current limit. The current limit can also be disabled by
floating the I
FEEDBACK LOOP/COMPENSATION
Feedback Loop Types
In a typical LTC3703 circuit, the feedback loop consists of
the modulator, the external inductor, the output capacitor
and the feedback amplifier with its compensation network.
All of these components affect loop behavior and must be
accounted for in the loop compensation. The modulator
consists of the internal PWM generator, the output MOSFET
drivers and the external MOSFETs themselves. From a
feedback loop point of view, it looks like a linear voltage
transfer function from COMP to SW and has a gain roughly
equal to the input voltage. It has fairly benign AC behavior
at typical loop compensation frequencies with significant
phase shift appearing at half the switching frequency.
The external inductor/output capacitor combination makes
a more significant contribution to loop behavior. These
components cause a second order LC roll off at the output,
with the attendant 180 phase shift. This rolloff is what
filters the PWM waveform, resulting in the desired DC
output voltage, but the phase shift complicates the loop
compensation if the gain is still higher than unity at the
pole frequency. Eventually (usually well above the LC pole
frequency), the reactance of the output capacitor will
approach its ESR and the rolloff due to the capacitor
will stop, leaving 6dB/octave and 90 of phase shift
(Figure 11).
MAX
pin.
DS(ON)
PROG
DS(ON)
of the MOSFETs. The maximum
voltage between 100mV and
, but not a minimum. A
LIMIT
which heats
DS(ON)
lies
3703f