uP1537

4

uP1537-DS-F0000, Feb. 2018

www.upi-semi.com

topology for the synchronous Buck. The uP1537 does not

extremely low ESR output capacitors and makes the

design easier and robust.

Enable and Soft Start

When the EN pin voltage rises above the enable threshold,

the controller enters its start up sequence. The first 250µs

calibrates the switching frequency setting resistance

attached at RF to GND and stores the switching frequency

code in internal registers. A voltage of 0.1V is applied to

RF for measurement. Switching is inhibited during this

period. In the second period, internal soft-start reference

voltage starts ramping up from 0V to 0.7V. This ramping

time is 750µs. Smooth and constant ramp up of the output

voltage is maintained during start up regardless of load

current. Connect a 1kΩ resistor in series with the EN pin

to provide protection.

On-Time Control and Frequency Setting

The uP1537 does not have a dedicated oscillator that

determines switching frequency. However, the device runs

with pseudo-constant frequency by feed-forwarding the

input and output voltages into its on-time one-shot timer.

proportional to the input voltage and proportional to the

output voltage. This makes the switching frequency fairly

constant in steady state conditions over wide input voltage

range. The switching frequency is selectable from four

preset values by a resistor connected to RF as shown in

Table 1. Leaving the RF pin open sets the switching

frequency to the lowest value, 290kHz, with Auto-skip

mode. However, it is recommended to apply one of the

resistances on the table in any application designs.

Table 1. Resistor and Switching Frequency

R

(

e

c

n

a

t

s

i

s

e

R

F

R

)

k

( Ω)

f

(

y

c

n

e

u

q

e

r

F

g

n

i

h

c

t

i

w

S

W

S

)

)

z

H

k

(

0

7

40

9

2

0

0

20

4

3

0

0

10

8

3

9

30

3

4

The off-time is modulated by a PWM comparator. The

VFB node voltage (the mid point of resistor divider) is

compared to the internal 0.7V reference voltage added

with a ramp signal. When both signals match, the PWM

comparator asserts the set signal to terminate the off-

Functional Description

time (turn off the low side MOSFET and turn on high side

MOSFET). The set signal becomes valid if the inductor

current level is below OCP threshold, otherwise the off-

time is extended until the current level becomes below

the threshold.

Light Load Condition in Auto-Skip Operation

When RF pin is pulled down to low via R

automatically reduces switching frequency at light load to

maintain high efficiency. This reduction of the frequency

is achieved smoothly and without increasing VOUT ripples

or load regulation. As the output current decreases from

heavy load condition, the inductor current will also be

reduced, and eventually comes to the point that its valley

touches zero current, which is the boundary between

continuous conduction and discontinuous conduction

modes. By emulating the behavior of diodes, the low side

MOSFET allows only partial of negative current when the

inductor freewheeling current reaches negative. As the

load current is further decreased, it takes longer time to

discharge the output capacitor to the level than requires

the next ON cycle.

The transition point from discontinuous to continuous

conduction mode can be calculated as:

)

V

V

1

(

V

L

f

2

1

I

IN

OUT

OUT

OUT

OSC

OUT

−

×

×

×

×

=

Forced Continuous Conduction Mode

When the RF pin is tied high, the controller keeps

continuous conduction mode (CCM) in light load condition.

In this mode, switching frequency is kept almost constant

over the entire load range which is suitable for applications

need tight control of the switching frequency at a cost of

lower efficiency. To set the switching frequency to be the

same as Auto-skip mode, it is recommended to connect

R

operation to set frequency and tied high after POK

indicates high.

Output Voltage Setting

Connect a resistor voltage-divider at the FB between

VOUT and GND to adjust the respective output voltage

between 0.7V and 2.6V. Choose R

using the equation as follows:

)

R

R

1

(

V

V

2

FB

1

FB

REF

OUT

+

×

=

where V