LM3151 datasheet(14/27 Pages) TI1

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VCL (Tj) = VCL x [1 + 3.3 x 10

-3

x (Tj - 27)]

ICL (Tj) =

VCL (Tj)

RDS(ON)max

LM3151, LM3152, LM3153

SNVS562G – SEPTEMBER 2008 – REVISED MARCH 2011

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Table 1. Inductor Selection Table (continued)

Inductor Designator

Inductance (µH)

Current (A)

Part Name

Vendor

L48

0.33

15-

4. Determine Output Capacitance

Typical hysteretic COT converters similar to the LM3151/2/3 require a certain amount of ripple that is generated

across the ESR of the output capacitor and fed back to the error comparator. Emulated Ripple Mode control built

into the LM3151/2/3 will recreate a similar ripple signal and thus the requirement for output capacitor ESR will

decrease compared to a typical Hysteretic COT converter. The emulated ripple is generated by sensing the

voltage signal across the low-side FET and is then compared to the FB voltage at the error comparator input to

determine when to initiate the next on-time period.

COmin = 70 / (fs

2 x L)

(1)

The maximum ESR allowed to prevent over-voltage protection during normal operation is:

ESRmax = (80 mV x L) / ETmin

ETmin is calculated using VIN-MIN

The minimum ESR must meet both of the following criteria:

ESRmin ≥ (15 mV x L) / ETmax

ESRmin ≥ [ETmax / (VIN - VOUT)]/ CO

ETmax is calculated using VIN-MAX.

Any additional parallel capacitors should be chosen so that their effective impedance will not negatively attenuate

the output ripple voltage.

5. MOSFET Selection

The high-side and low-side FETs must have a drain to source (VDS) rating of at least 1.2 x VIN.

The gate drive current from VCC must not exceed the minimum current limit of VCC. The drive current from VCC

can be calculated with:

IVCCdrive = Qgtotal x fS

where

•

Qgtotal is the combined total gate charge of the high-side and low-side FETs

Use the following equations to calculate the current limit, ICL, as shown in Figure 12.

where

•

Tj is the junction temperature of the LM3151/2/3

The plateau voltage of the FET VGS vs Qg curve, as shown in Figure 14 must be less than VCC - 750 mV.

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