VCC

20 x

LM5035C

CS

LO

AGND

R1

HO

Q1

Q2

Current

Sense

Power

Transformer

VIN

LM5035C

www.ti.com

SNVS631C – JANUARY 2010 – REVISED MARCH 2013

Figure 21. Current Sense Using Current Sense Resistor (R1)

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 for continuous cycle-by-cycle current limiting 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 time average of this toggling is equivalent to a continuous 5 µA current source into the RES capacitor,

increasing the delay by a factor of four. 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 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 topology operated with cycle-by-cycle current limiting and is compounded by only sensing in

one leg of the half-bridge circuit. The imbalance is greatest at large duty cycles (low input voltages). If using this

method, it is recommended that the capacitor on the RES pin be no larger than 220 pF. Check the final circuit

and reduce the RES capacitor further, or omit the capacitor completely to ensure the voltages across the bridge

capacitors remain balanced. The current limit value may decrease slightly as the RES capacitor is reduced.

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 LM5035C, 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 discharging to the point

(6)

of withstanding 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 voltage because the high-side driver is

boosted up to the converter input voltage by the HS pin, and the high side MOSFET gate is driven to the HS

applications, so a low current ultra-fast recovery diode is recommended 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 leakage currents at high temperatures.

The internal gate drivers need a very low impedance path to the respective decoupling capacitors; the VCC cap

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.

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