CS51312 datasheet(9/18 Pages) CHERRY

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Application Information: continued

When driving large capacitive loads, the COMP must

charge slowly enough to avoid tripping the CS51312 over-

current protection. The following equation can be used to

ensure unconditional startup:

where

ICHG = COMP Source Current (30µA typical);

CCOMP = COMP Capacitor value (0.1µF typical);

ILIM = Current Limit Threshold;

ILOAD = Load Current during startup;

COUT = Total Output Capacitance.

Normal Operation

During Normal operation, Switch Off-Time is constant and

set by the COFF capacitor. Switch On-Time is adjusted by

the V2TM Control loop to maintain regulation. This results

in changes in regulator switching frequency, duty cycle,

and output ripple in response to changes in load and line.

Output voltage ripple will be determined by inductor rip-

ple current and the ESR of the output capacitors

Transient Response

The CS51312 V2

TM Control Loop’s 200ns reaction time pro-

vides unprecedented transient response to changes in

input voltage or output current. Pulse-by-pulse adjustment

of duty cycle is provided to quickly ramp the inductor cur-

rent to the required level. Since the inductor current cannot

be changed instantaneously, regulation is maintained by

the output capacitor(s) during the time required to slew the

inductor current.

Overall load transient response is further improved

through a feature called “Adaptive Voltage Positioning”.

This technique pre-positions the output capacitors voltage

to reduce total output voltage excursions during changes

in load.

Holding tolerance to 1% allows the error amplifiers refer-

ence voltage to be targeted +25mV high without compro-

mising DC accuracy. A “Droop Resistor” connects the

Error Amps feedback pin (VFB) to the output capacitors

and load and carries the output current. With no load,

there is no DC drop across this resistor, producing an out-

put voltage tracking the Error amps, including the +25mV

offset. When the full load current is delivered, a 50mV

drop is developed across this resistor. This results in out-

put voltage being offset -25mV low.

The benefit of Adaptive Voltage Positioning is that addi-

tional margin is provided for a load transient before reach-

ing the output voltage specification limits. When load cur-

rent suddenly increases from its minimum level, the out-

put capacitor is pre-positioned +25mV. Conversely, when

load current suddenly decreases from its maximum level,

the output capacitor is pre-positioned -25mV. For best

Transient Response, a combination of a number of high fre-

quency and bulk output capacitors are usually used.

Protection and Monitoring Features

Over-Current Protection

A hiccup mode current limit protection feature is provided,

requiring only the COMP capacitor to implement. The

CS51312 provides overcurrent protection by sensing the

current through a “Droop” resistor, using an internal cur-

rent sense comparator. The comparator compares the volt-

age drop through the “Droop” resistor to an internal refer-

ence voltage of 86mV (typical).

If the voltage drop across the “Droop” resistor exceeds this

threshold, the current sense comparator allows the fault

latch to be set. This causes the regulator to stop switching.

During this over current condition, the CS51312 stays off

for the time it takes the COMP pin capacitor to discharge

to its lower 0.25V threshold. As soon as the COMP pin

reaches 0.25V, the Fault latch is reset (no overcurrent con-

dition present) and the COMP pin is charged with a 30µA

current source to a voltage 1.1V greater than the VFB volt-

age. Only at this point the regulator attempts to restart nor-

mally by delivering short gate pulses to both FETS. This

protection scheme minimizes thermal stress to the regula-

tor components, input power supply, and PC board traces,

as the over current condition persists. Upon removal of the

overload, the fault latch is cleared, allowing normal opera-

tion to resume.

Overvoltage Protection

Overvoltage protection (OVP) is provided as result of the

normal operation of the V2

TM control topology and requires

no additional external components. The control loop

responds to an overvoltage condition within 200ns, causing

the top MOSFET to shut off, disconnecting the regulator

from its input voltage. This results in a “crowbar” action to

clamp the output voltage and prevents damage to the load.

The regulator will remain in this state until the overvoltage

condition ceases or the input voltage is pulled low.

Additionally, a dedicated Overvoltage protection (OVP)

output pin (pin 13) is provided in the CS51312. The OVP

signal will go high (overvoltage condition), if the output

voltage (VCC(CORE)) exceeds the regulation voltage by 8.5%

of the voltage set by the particular DAC code. The OVP pin

can source up to 25mA of current that can be used to drive

an SCR to crowbar the power supply.

Power-Good Circuit

The Power-Good pin (pin 14) is an open-collector signal

consistent with TTL DC specifications. It is externally

pulled up, and is pulled low (below 0.3V) when the regula-

tor output voltage typically exceeds ± 8.5% of the nominal

output voltage. Maximum output voltage deviation before

Power-Good is pulled low is ± 12%.

Output Enable

On/off control of the regulator outputs can be implement-

ed by pulling the COMP pins low. It is required to pull the

COMP pins below the 1.1V PWM comparator offset volt-

age in order to disable switching on the GATE drivers.

ILIM − ILOAD

COUT

ICHG

CCOMP

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