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ML4790

FUNCTIONAL DESCRIPTION

The ML4790 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 ML4790 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, the offset voltage control, and

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.

450

400

350

300

250

200

150

100

0

10

20

30

40

50

60

70

80

90 100

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.

+

–

6

–

5

µs

ONE SHOT

R

S

L1

Q3

A2

A1

Q1

R1

+

–

+

5

C2

+

–

A3

R2

4

Q2

3

Figure 2. PFM Regulator and LDO Block Diagram