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CS5111

14.1A switching regulator with 5V,100mA linear regulator with watchdog,reset and enable

Cherry semiconductor 

Circuit Description: continued

If a correct watchdog signal is not received within the

specified time a reset pulse train is issued until the correct

watchdog signal is received. The nominal reset signal in

this case is a 5 volt square wave with a 50% duty cycle as

shown in Figure 4.

The RESET signal frequency is given by:

fRESET =

1

2(tWDI)

The Power On Reset (POR) and low voltage RESET use

the same circuitry and issue a reset when the linear output

voltage is below the regulation limit. After VLIN rises

above the minimum specified value, RESET remains low

for a fixed period tPOR as shown in Figure 5.

The POR delay (tPOR) is given by:

tPOR = 1.353 × CDelay RBIAS

VLIN

4.45V

4.25V

RESET

VRPEAK

VRLO

tPOR

Figure 5a. The power on reset time interval (tPOR) begins when VLIN

rises above 4.45V (typical).

VLIN

5V

4.25V

RESET

5V

tPOR

Figure 5b. RESET signal is issued whenever VLIN falls below 4.25V

(typical).

Current Mode PWM Switching Circuitry

The current mode PWM switching voltage regulator con-

tains an error amplifier with selectable feedback inputs, a

current sense amplifier, an adjustable oscillator and a 1.4A

output power switch with antisaturation control. The

switching regulator and external components, connected

in a boost configuration, are shown in Figure 6.

The switching regulator begins operation when VREG and

VIN are raised above 5 volts. VREG is required since the

switching supply’s control circuitry is powered through

VLIN. VIN supplies the base drive to the switcher output

transistor.

The output transistor turns on when the oscillator starts to

charge the capacitor on COSC. The output current will

develop a voltage drop across the internal sense resistor

(RS). This voltage drop produces a proportional voltage at

the output of the current sense amplifier, which is com-

pared to the output of the error amplifier. The error ampli-

fier generates an output voltage which is proportional to

the difference between the scaled down output boost volt-

age (VFB1 or VFB2) and the internal bandgap voltage refer-

ence. Once the current sense amplifier output exceeds the

error amplifier’s output voltage, the output transistor is

turned off.

The energy stored in the inductor during the output tran-

sistor on time is transferred to the load when the output

transistor is turned off. The output transistor is turned

back on at the next rising edge of the oscillator. On a cycle

by cycle basis, the current mode controller in a discontinu-

ous mode of operation charges the inductor to the appro-

priate amount of energy, based on the energy demand of

the load. Figure 7 shows the typical current and voltage

waveforms for a boost supply operating in the discontinu-

ous mode.

NOTES:

1. Refer to Figure 1d to determine oscillator frequency.

2. The switching regulator can be disabled by providing a

logic high at the ENABLE input.

3. The boost output voltage can be controlled dynamically

by the feedback select input. If select is open, VFB2 is

selected. If select is low, then VFB1 is selected.

Protection Circuitry

If the input voltage at VREG is increased above the over-

voltage threshold, the drive to the linear and switcher out-

put transistors is shut off. Therefore, VLIN is disabled and

VSW can not be pulled low.

The current out of VLIN is sensed in order to limit exces-

sive power dissipation in the linear output transistor over

the output range of 0V to regulation. Also, the current into

VSW is sensed in order to provide the current limit func-

tion in the switcher output transistor.

If the die temperature is increased above 160°C, either due

to excessive ambient temperature or excessive power dis-

sipation, the drive to the linear output transistor is

reduced proportionally with increasing die temperature.

Therefore, VLIN will decrease with increasing die tempera-

ture above 160°C. Since the switcher control circuitry is

powered through VLIN, the switcher performance, includ-

ing current limit, will be affected by the decrease in VLIN.

6

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