Texas Instruments TPS40003 manual Snubber Component Selection

SLUU130A – September 2002 – Revised February 2003

Figure 2 shows the plots for the closed loop gain and phase with VIN = 3.3 V and IOUT = 4.4 A. At the crossover frequency of 60 kHz the phase margin is approximately 51 degrees.

GAIN AND PHASE MARGIN

			vs
			FREQUENCY
	40				150
	30			Phase	100
	20
	10				50	degrees
– db
	0				0
Gain						Phase –
	–10				–50
			Gain
	–20
	–30	ILOAD = 4.4 A			–100
		VI = 3.3 V
	–40				–150
	100	1000	10000	100000	1000000

Frequency – Hz

Figure 2.

4.8Snubber Component Selection

The switch node where Q1 and L1 come together is very noisy. An R–C network fitted between this node and ground can help reduce ringing and voltage overshoot on Q1:B. This ringing noise should be minimized to prevent it from confusing the control circuitry which is monitoring this node for current limit and Predictive Gate Drive .

As a starting point, the snubber capacitor, C9, is generally chosen to be 5 to 8 times larger than the parasitic capacitance at the node, which is primarily COS of Q1:B. Since COS is 440 pF for Q1:B, C9 is chosen to be 3.3 nF. R3 is empirically determined to be 2.2 Ω , which minimizes the ringing and overshoot at the switch node. With low input voltages the power loss, 1/2⋅ C⋅ V2⋅ f , is relatively small at 24 mW.

TPS40003-Based 5-A Converter in Less Than One Square Inch 9