until VCC rises above 4.2V. As with shutdown, the soft start
capacitor is discharged through a FET, ensuring that the next
start-up will be smooth.
Application Information (Continued)
CURRENT LIMIT
Current limit is realized by sensing the voltage across the
low side FET while it is on. The RDSON of the FET is a known
value, hence the current through the FET can be determined
as:
MOSFET GATE DRIVERS
The LM2727/37 has two gate drivers designed for driving
N-channel MOSFETs in a synchronous mode. Power for the
drivers is supplied through the BOOTV pin. For the high side
gate (HG) to fully turn on the top FET, the BOOTV voltage
must be at least one VGS(th) greater than Vin. (BOOTV ≥
2*Vin) This voltage can be supplied by a separate, higher
voltage source, or supplied from a local charge pump struc-
ture. In a system such as a desktop computer, both 5V and
12V are usually available. Hence if Vin was 5V, the 12V
supply could be used for BOOTV. 12V is more than 2*Vin, so
the HG would operate correctly. For a BOOTV of 12V, the
initial gate charging current is 2A, and the initial gate dis-
charging current is typically 6A.
VDS = I * RDSON
The current limit is determined by an external resistor, RCS
,
connected between the switch node and the ISEN pin. A
constant current of 50µA is forced through Rcs, causing a
fixed voltage drop. This fixed voltage is compared against
VDS and if the latter is higher, the current limit of the chip has
been reached. RCS can be found by using the following:
RCS = RDSON(LOW) * ILIM/50µA
For example, a conservative 15A current limit in a 10A
design with a minimum RDSON of 10mΩ would require a
3.3kΩ resistor. Because current sensing is done across the
low side FET, no minimum high side on-time is necessary. In
the current limit mode the LM2727/37 will turn the high side
off and the keep low side on for as long as necessary. The
chip also discharges the soft start capacitor through a fixed
95µA source. In this way, smooth ramping up of the output
voltage as with a normal soft start is ensured. The output of
the LM2727/37 internal error amplifier is limited by the volt-
age on the soft start capacitor. Hence, discharging the soft
start capacitor reduces the maximum duty cycle D of the
controller. During severe current limit, this reduction in duty
cycle will reduce the output voltage, if the current limit con-
ditions lasts for an extended time.
20049402
During the first few nanoseconds after the low side gate
turns on, the low side FET body diode conducts. This causes
an additional 0.7V drop in VDS. The range of VDS is normally
much lower. For example, if RDSON were 10mΩ and the
current through the FET was 10A, VDS would be 0.1V. The
current limit would see 0.7V as a 70A current and enter
current limit immediately. Hence current limit is masked dur-
ing the time it takes for the high side switch to turn off and the
low side switch to turn on.
FIGURE 1. BOOTV Supplied by Charge Pump
In a system without a separate, higher voltage, a charge
pump (bootstrap) can be built using a diode and small ca-
pacitor, Figure 1. The capacitor serves to maintain enough
voltage between the top FET gate and source to control the
device even when the top FET is on and its source has risen
up to the input voltage level.
UVP/OVP
The LM2727/37 gate drives use a BiCMOS design. Unlike
some other bipolar control ICs, the gate drivers have rail-to-
rail swing, ensuring no spurious turn-on due to capacitive
coupling.
The output undervoltage protection and overvoltage protec-
tion mechanisms engage at 70% and 118% of the target
output voltage, respectively. In either case, the LM2727 will
turn off the high side switch and turn on the low side switch,
and discharge the soft start capacitor through a MOSFET
switch. The chip remains in this state until the shutdown pin
has been pulled to a logic low and then released. The UVP
function is masked only during the first charging of the soft
start capacitor, when voltage is first applied to the VCC pin. In
contrast, the LM2737 is designed to continue operating dur-
ing UVP or OVP conditions, and to resume normal operation
once the fault condition is cleared. As with the LM2727, the
powergood flag goes low during this time, giving a logic-level
warning signal.
POWER GOOD SIGNAL
The power good signal is the or-gated flag representing
over-voltage and under-voltage protection. If the output volt-
age is 18% over it’s nominal value, VFB = 0.7V, or falls 30%
below that value, VFB = 0.41V, the power good flag goes low.
The converter then turns off the high side gate, and turns on
the low side gate. Unlike the output (LM2727 only) the power
good flag is not latched off. It will return to a logic high
whenever the feedback pin voltage is between 70% and
118% of 0.6V.
SHUT DOWN
UVLO
If the shutdown pin SD is pulled low, the LM2727/37 dis-
charges the soft start capacitor through a MOSFET switch.
The high side switch is turned off and the low side switch is
turned on. The LM2727/37 remains in this state until SD is
released.
The 4.2V turn-on threshold on VCC has a built in hysteresis
of 0.6V. Therefore, if VCC drops below 3.6V, the chip enters
UVLO mode. UVLO consists of turning off the top FET,
turning on the bottom FET, and remaining in that condition
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