ML4890CS-3 datasheet(5/12 Pages) MICRO-LINEAR

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ML4890

Also included in the LDO stage is an offset voltage

control. This circuit monitors the output current and

adjusts the offset voltage according the general

characteristic shown in Figure 3. The offset control

ensures that the PFM stage provides just enough

“overhead” voltage for the LDO stage to operate properly.

PFM REGULATOR OPERATION

When the output of the PFM stage, VBOOST (pin 5), is at or

above the dropout voltage, VOUT + VOS, the output of A1

stays low and the circuit remains idle. When VBOOST falls

below the required dropout voltage, the output of A1 goes

high, signaling the regulator to deliver charge to the

capacitor C2. Since the output of A2 is normally high, the

output of the flip-flop becomes SET. This triggers the one

shot to turn Q1 on and begins charging L1 for 5

µs. When

the one shot times out, Q1 turns off, allowing L1 to

flyback and momentarily charge C2 through the body

diode of Q2. But, as the source voltage of Q2 rises above

the drain, the current sensing amplifier A2 drives the gate

of Q2 low, causing Q2 to short out the body diode. The

inductor then discharges into C2 through Q2. The output

of A2 going low also serves to RESET the flip-flop in

preparation for the next charging cycle. When the

inductor current in Q2 falls to zero, the output of A2 goes

high, releasing Q2‘s gate, allowing the flip-flop to be SET

again. If the voltage at VBOOST is still low, A1 will initiate

another pulse. Typical inductor current and voltage

waveforms are shown in Figure 4.

Q(ONE SHOT)

Q1 ON

INDUCTOR

CURRENT

Q1 & Q2 OFF

Figure 4. PFM Inductor Current Waveforms and Timing.

SHUTDOWN

The SHDN pin should be held low for normal operation.

Raising the voltage on SHDN above the threshold level

will release the gate of Q3, which effectively becomes an

open circuit. This also prevents the one shot from

triggering, which keeps switching from occurring.

FUNCTIONAL DESCRIPTION

The ML4890 combines Pulse Frequency Modulation

(PFM) and synchronous rectification to create a boost

converter that is followed by a low dropout linear

regulator (LDO). This combination creates a low output

ripple boost converter that is both highly efficient and

simple to use.

The PFM regulator charges a single inductor for a fixed

period of time and then completely discharges before

another cycle begins, simplifying the design by

eliminating the need for conventional current limiting

circuitry. Synchronous rectification is accomplished by

replacing an external Schottky diode with an on-chip

PMOS device, reducing switching losses and external

component count.

The integrated LDO reduces the output ripple voltage to

less than 5mV peak-to-peak. Integrating the LDO along

with the PFM regulator allows the circuit to be optimized

for very high efficiency using a patented feedback

technique. It also allows the LDO to provide the

maximum ripple rejection over the operating frequency

range of the regulator.

A block diagram of the ML4890 is shown in Figure 2. The

PFM stage is comprised of Q1, Q2, A1, A2, the one shot,

the flip-flop, and externals L1 and C2. The LDO stage is

comprised of Q3, A3, R1, R2, the offset voltage control,

and external C1. Since the LDO actually controls the

operation of the PFM regulator, the operation of the LDO

stage will be covered first.

LDO OPERATION

The LDO stage operates as a linear regulator. A3 is the

error amplifier, which compares the output voltage

through the divider R1 and R2 to the reference, and Q3 is

the pass device. When the output voltage is lower than

desired, the output of A3 increases the gate drive of Q3,

which reduces the voltage drop across it and brings the

output back into regulation. Similarly, if the output voltage

is higher than desired, A3 adjusts the gate drive of Q3 for

more drop and the output is brought back into regulation.

Figure 3. LDO VOS versus output current.

450

400

350

300

250

200

150

100

IOUT (mA)

10 20 30 40 50 60 70 80 90 100

ML4890CS-3 Datasheet(PDF) 5 Page - Micro Linear Corporation

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