lm5039mhx National Semiconductor Corporation, lm5039mhx Datasheet - Page 18

• Manufacturer Part Number: lm5039mhx
• Description: Half-bridge Pwm Controller With Average Current Limit
• Manufacturer: National Semiconductor Corporation

www.national.com
If the current sense resistor method is used, the over-current
condition will only be sensed while LO is driving the low-side
MOSFET. Over-current while HO is driving the high-side
MOSFET will not be detected. In this configuration, it will take
4 times as long to initiate a restart event since each over-
current event during LO enables the 22µA RES pin current
source for one oscillator period, and then the lack of an over-
current event during HO enables the 12µA RES pin current
sink for one oscillator period. The value of the RES capacitor
can be reduced to decrease the time before restart cycle is
initiated.
When using the resistor current sense method, an imbalance
in the input capacitor voltages may develop when operating
in peak cycle-by-cycle current limiting mode. If the imbalance
persists for an extended period, excessive currents in the
non-sensed MOSFET, and possible transformer saturation
may result. This condition is inherent to the half-bridge topol-
ogy operated with peak cycle-by-cycle current limiting and is
compounded by only sensing in one leg of the half-bridge cir-
cuit. The imbalance is greatest at large duty cycles (low input
voltages). It is recommended to activate average current limit
circuitry in such a configuration. However, since only alterna-
tive cycles source current into the ACL capacitor, ACL ca-
pacitor needs to be halved. This could still lead to a slight
imbalance depending upon the input/output voltage levels
and the impedance mismatch between the two phases of the
half-bridge due to the additional CS resistor in the bottom half.
HO, HB, HS and LO
Attention must be given to the PC board layout for the low-
side driver and the floating high-side driver pins HO, HB and
HS. A low ESR/ESL capacitor (such as a ceramic surface
mount capacitor) should be connected close to the LM5039,
between HB and HS to provide high peak currents during turn-
on of the high-side MOSFET. The capacitor should be large
enough to supply the MOSFET gate charge (Qg) without dis-
charging to the point where the drop in gate voltage affects
the MOSFET R
ommended.
DS(ON)
. A value ten to twenty times Qg is rec-
FIGURE 9. Current Sense Using Current Sense Resistor (R1)
18
The diode (D
low-side MOSFET is conducting should be capable of with-
standing the full converter input voltage range. When the
high-side MOSFET is conducting, the reverse voltage at the
diode is approximately the same as the MOSFET drain volt-
age because the high-side driver is boosted up to the con-
verter input voltage by the HS pin, and the high side MOSFET
gate is driven to the HS voltage plus VCC. Since the anode
of D
the diode is equal to the input voltage minus the VCC voltage.
D
tions, so a low current ultra-fast recovery diode is recom-
mended to limit the loss due to diode junction capacitance.
Schottky diodes are also a viable option, particularly for lower
input voltage applications, but attention must be paid to leak-
age currents at high temperatures.
The internal gate drivers need a very low impedance path to
the respective decoupling capacitors; the VCC cap for the LO
driver and C
should be as short as possible to reduce inductance and as
wide as possible to reduce resistance. The loop area, defined
by the gate connection and its respective return path, should
be minimized.
The high-side gate driver can also be used with HS connected
to PGND for applications other than a half bridge converter
(e.g. Push-Pull). The HB pin is then connected to VCC, or any
supply greater than the high-side driver undervoltage lockout
(approximately 6.5V). In addition, the high-side driver can be
configured for high voltage offline applications where the
high-side MOSFET gate is driven via a gate drive transformer.
BOOST
BOOST
average current is less than 20mA in most applica-
is connected to VCC, the reverse potential across
BOOST
BOOST
) that charges C
for the HO driver. These connections
30100525
BOOST
from VCC when the