LM2657 datasheet(12/26 Pages) NSC

12 / 26 page

Operation Descriptions (Continued)

START-UP AND STATE-TRANSITIONS AT LIGHT

LOADS

During startup the LM2657 is allowed to operate in SKIP

mode regardless of the voltage on the FPWM pin. Starting in

SKIP mode prevents the low-side FET from having to sink

excessive amounts of negative current during start-up. This

would occur if the output was pre-biased and the converter

operated in FPWM mode. The FB pin would sense that the

output was high and force a very low duty cycle, which would

keep the low-side FET on longer than it should be in steady

state. Without a load on the output the inductor current will

reverse and become negative. This negative inductor current

can be quite large depending on the voltage on the output

and the size of the output capacitor.

A similar situation can occur if the converter transitions from

SKIP mode to FPWM mode under a light load condition

(converter is operating below the DCM boundary). This can

occur after startup if FPWM mode is selected for use in a

light load condition or if the FPWM pin is toggled high during

normal operation at light load. The problem occurs because

in SKIP mode the converter is operating at a set duty cycle

and a lower average frequency. When the converter is

forced into FPWM mode, this represents a change to the

system. The pulse widths and frequency need to readjust

suddenly and in the process momentary imbalances can be

created. Like the case of a pre-biased load, there can be

negative surge current passing from drain to source of the

lower FET. It must be kept in mind that though the LM2657

has current limiting for current passing in the ‘positive’ direc-

tion (positive with regards to the inductor, i.e. passing from

source to drain of the lower FET), there is no limit for reverse

currents. The amount of reverse current when the FPWM pin

is toggled ‘on the fly’ can be very high. This current is

determined by several factors. One key factor is the output

capacitance. Large output capacitances will lead to higher

peak reverse currents. The reverse swing will be higher for

lighter loads because of the bigger difference between the

duty cycles/average frequency in the two modes. See Figure

5 for a plot of what happened in going from SKIP to FPWM

mode at 0A load (worst case). The peak reverse current was

as high as 3A, lasting about 0.1ms. The inductor could also

saturate severely at this point if designed for light loads. In

general, if the designer wants to toggle the FPWM pin while

the converter is operating or if FPWM mode is required for a

light load application, the low side FET and inductor should

be closely evaluated under this specific condition.

If the part is operated in FPWM mode with a light load the

user will experience lower efficiency and negative current

during the transition (as discussed). The user may also

experience a momentary drop on Vout when the transition is

made from SKIP to FPWM mode. This only occurs for no

load or very light load conditions (above the DCM boundary

there is no difference between SKIP FPWM modes).

In some cases, such as low Vout (<1.5V), a glitch may be

present on PGOOD. If this is problematic, the glitch may be

eliminated by either operating in SKIP mode or using a small

sized soft start capacitor. See the following section for se-

lecting soft start capacitors.

SOFT-START

The maximum output voltage of the error amplifier is limited

during start-up by the voltage on the 0.1µF capacitor con-

nected between the SS pin and ground. When the controller

is enabled (by taking EN pin high), the following steps may

occur. First, the SS capacitor is discharged (if it has a

pre-charge) by a 1.8 k

Ω internal resistor (R

SS_DCHG, see

Electrical Characteristics). This ensures that reset is ob-

tained. Note that reset is said to occur only when the voltage

20134712

FIGURE 4. Inductor Current in FPWM Mode

20134713

CH1: PGOOD, CH2: Vo, CH3: LDRV, CH4: IL (1A/div)

Output 1V @ 0A, VIN = 10V, L = 10µH, f = 300kHz

FIGURE 5. SKIP to FPWM ’On The Fly’

LM2657 Datasheet(PDF) 12 Page - National Semiconductor (TI)

LM2657 Datasheet(HTML) 12 Page - National Semiconductor (TI)

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26

Datasheet Download

Link URL