lm2737mtcx National Semiconductor Corporation, lm2737mtcx Datasheet - Page 10

• Manufacturer Part Number: lm2737mtcx
• Description: N-channel Fet Synchronous Buck Regulator Controller For Low Output Voltages
• Manufacturer: National Semiconductor Corporation

www.national.com
Application Information
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 V
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.
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.
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.
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, V
below that value, V
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.
UVLO
The 4.2V turn-on threshold on V
of 0.6V. Therefore, if V
UVLO mode. UVLO consists of turning off the top FET,
turning on the bottom FET, and remaining in that condition
FIGURE 1. BOOTV Supplied by Charge Pump
FB
= 0.41V, the power good flag goes low.
CC
GS(th)
drops below 3.6V, the chip enters
greater than Vin. (BOOTV ≥
CC
has a built in hysteresis
FB
= 0.7V, or falls 30%
20049402
(Continued)
10
until V
capacitor is discharged through a FET, ensuring that the next
start-up will be smooth.
CURRENT LIMIT
Current limit is realized by sensing the voltage across the
low side FET while it is on. The R
value, hence the current through the FET can be determined
as:
The current limit is determined by an external resistor, R
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
V
been reached. R
For example, a conservative 15A current limit in a 10A
design with a minimum R
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.
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 V
much lower. For example, if R
current through the FET was 10A, V
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.
UVP/OVP
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 V
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.
SHUT DOWN
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.
DS
and if the latter is higher, the current limit of the chip has
CC
rises above 4.2V. As with shutdown, the soft start
R
CS
CS
= R
can be found by using the following:
V
DSON
DS
= I * R
DSON
(LOW) * I
DS
. The range of V
DSON
DSON
DSON
of 10mΩ would require a
DS
LIM
of the FET is a known
were 10mΩ and the
/50µA
would be 0.1V. The
DS
is normally
CC
pin. In
CS
,