MIC24066 datasheet(21/38 Pages) MICROCHIP

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DS20006730A-page 21

MIC24066/7

FIGURE 4-6:

Output Capacitive and ESR

Ripple Vector Relation.

External injected ripple is not required when ESR ripple

is high as compared to capacitive ripple. The modern

loads require output voltage steady state ripple to be

less than 1% of regulation voltage. For output voltages

in the range of 1V, output steady state ripple voltage

requirement is in the order of 10 mV for modern high

current ASIC loads. Customers generally use ceramic

capacitors or low ESR tantalum capacitors to meet

steady state requirements for their loads. In those

cases, MIC24066/7 control loop requires external

ripple injection to the FB node.

Note that as RBIAS is always present, it draws an

additional current form the injection driver when INJ pin

is 4.5V for 100 ns. This adds to the device's IQ.

However, its contribution to the device's IQ will be low,

because this current will be present for 100 ns only.

Another thing to note is that the INJ driver must be

capable of supplying this additional current.

4.3

Ripple Injection for Low ESR

Capacitors

When customers use low ESR electrolytic capacitors or

tantalum capacitors, external ripple can be injected by

connecting feed forward capacitor (CFF) from output to

the FB node as shown in Figure 4-7.

FIGURE 4-7:

Feed Forward Capacitor.

EQUATION 4-3:

As shown in Figure 4-8, feed forward capacitor

bypasses high frequency ripple to the FB pin and

provides phase boost at mid frequency.

FIGURE 4-8:

Feed Forward Capacitor

Gain and Phase Plots.

4.4

Ripple Injection for Ceramic

Capacitors

Customers use ceramic capacitors as output filter for

much high performance ASIC. Ceramic capacitors

have very low ESR and ESL. The MIC24066/7 need

extra ripple injected from switch node. In those

applications, external ripple can be injected by

connecting series RC network from switch node to FB

node as shown in Figure 4-9.

FIGURE 4-9:

MIC24066/7 External Ripple

Injection Circuit.

The purpose of CB is to block the DC component and

forward high frequency from the switch node. It is

required to select CB value very much higher than CFF

value. The following equation denotes the amount of

ripple injected at FB node.

No External Ripple

injection is needed

for this kind of

application

VESR

VFB

VCO

VFB

VOUT

-------------(s)

---------------------

1s CFF





1s CFF



R1 R2



---------------------







--------------------------------------------------------------



RINJ

Comp

0.6V

TON

CFF

RINJ

Comp

0.6V

TON

CFF

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