LTC3703
applicaTions inForMaTion
Bottom MOSFET Source Supply (BGRTN)
ambient temperature, conditions that cause the largest
power loss in the converter. Note that it is important to
checkforself-consistencybetweentheassumedMOSFET
The bottom gate driver, BG, switches from DRV to
CC
BGRTN 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 threshold of the FET, shoot-through will oc-
cur. By using a negative supply on BGRTN, the BG can be
pulledbelowgroundwhenturningthebottomMOSFEToff.
This provides a few extra volts of margin before the gate
reaches the turn-on threshold of the MOSFET. Be aware
junctiontemperatureandtheresultingvalueofI
heats the MOSFET switches.
which
LIMIT
Cautionshouldbeusedwhensettingthecurrentlimitbased
upon the R of the MOSFETs. The maximum current
DS(ON)
limitisdeterminedbytheminimumMOSFETon-resistance.
Datasheetstypicallyspecifynominalandmaximumvalues
for R
, but not a minimum. A reasonable assumption
DS(ON)
is that the minimum R
lies the same amount below
DS(ON)
the typical value as the maximum lies above it. Consult the
MOSFET manufacturer for further guidelines.
For best results, use a V
voltage between 100mV and
PROG
that the maximum voltage difference between DRV and
CC
500mV. Values outside of this range may give less accu-
BGRTNis15V.If,forexample,V
=–2V,themaximum
BGRTN
rate current limit. The current limit can also be disabled
voltage on DRV pin is now 13V instead of 15V.
CC
by floating the I
pin.
MAX
Current Limit Programming
FEEDBACK LOOP/COMPENSATION
Feedback Loop Types
Programming current limit on the LTC3703 is straight
forward. The I
pin sets the current limit by setting
MAX
the 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
limitcircuitinvertsthenegativevoltageacrosstheMOSFET
In a typical LTC3703 circuit, the feedback loop consists of
the modulator, the external inductor, the output capacitor
andthefeedbackamplifierwithitscompensationnetwork.
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 MOS-
FET drivers and the external MOSFETs themselves. From
a feedback loop point of view, it looks like a linear voltage
transferfunctionfromCOMPtoSWandhasagainroughly
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.
before comparing it to the voltage at I
current limit to be set with a positive voltage.
, allowing the
MAX
To set the current limit, calculate the expected voltage
drop across the bottom MOSFET at the maximum desired
current and maximum junction temperature:
V
= (I
)(R )(1 + δ)
DS(ON)
PROG
LIMIT
where δ is explained in the MOSFET Selection section.
is then programmed at the I pin using the
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
V
PROG
MAX
internal 12µA pull-up and an external resistor:
R
IMAX
= V /12µA
PROG
The current limit value should be checked to ensure
that I > I and also that I is
LIMIT(MIN)
OUT(MAX)
LIMIT(MAX)
less than the maximum rated current of the inductor
and bottom MOSFET. The minimum value of current
limit generally occurs with the largest V at the highest
IN
3703fc
18
Linear Systems [ Linear Systems ]
