Datasheet CS5157GDR16, CS5157GD16 Datasheet (Cherry Semiconductor)

The CS5157 is a 5-bit synchronous
dual N-Channel buck controller. It
is designed to provide unprece­dented transient response for
today’s demanding high-density,
high-speed logic. The regulator
operates using a proprietary control
method, which allows a 100ns
response time to load transients.
The CS5157 is designed to operate
over a 4.25-16V range (VCC) using
12V to power the IC and 5V as the
main supply for conversion.

The CS5157 is specifically designed
to power Pentium®II processors
and other high performance core
logic. It includes the following fea­tures: on board, 5-bit DAC, short
circuit protection, 1.0% output tol­erance, VCCmonitor, and pro­grammable soft start capability. The
CS5157 is available in 16 pin surface
mount.

Features

■ Dual N-Channel Design

■ Excess of 1MHz Operation

■ 100ns Transient Response

■ 5-Bit DAC

■ Backward Compatible with

Adjustable CS5120/5121

■ 30ns Gate Rise/Fall Times

■ 1% DAC Accuracy

■ 5V & 12V Operation

■ Remote Sense

■ Programmable Soft Start

■ Lossless Short Circuit

Protection

■ V

CC

Monitor

■ 25ns FET Nonoverlap Time

■ V

2

TM

Control Topology

■ Current Sharing

■ Overvoltage Protection

Package Options

CPU 5-Bit Synchronous Buck Controller

CS5157

Description

Application Diagram

V

ID0

V

ID1

V

ID2

V

ID3

SS

V

ID4

C

OFF

V

FFB

V

FB

COMP

LGnd

V

CC1

V

GATE(L)

PGnd
V

GATE(H)

V

CC2

16 Lead SO Narrow

1

Switching Power Supply for core logic - Pentium®II processor

0.33µF

V

ID0

V

ID1

V

ID2

V

ID3

V

ID0

V

ID1

V

ID2

V

ID3

V

CC1

SS

CS5157

C

OFF

LGnd

V

FB

V

FFB

COMP

IRL3103

IRL3103

0.1µF

12V 5V

2µH

1.3V to 3.5V @ 13A

V

CC2

V

GATE(H)

V

GATE(L)

PGnd

1200µF/10V x 3
AlEl

3.3k

0.1µF

1200µF/10V x 5
AlEl

100pF

330pF

V

ID4

V

ID4

Pentium is a registered trademark of Intel Corporation.

V2is a trademark of Switch Power, Inc.

Rev. 12/31/98

CS5157

Cherry Semiconductor Corporation

2000 South County Trail, East Greenwich, RI 02818

Tel: [phone] Fax: [phone]

Email: [e-mail]

Web Site: www.cherry-semi.com

A Company

®

1

2

Pin Name Max Operating Voltage Max Current

V

CC1

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .16V/-0.3V . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .25mA DC/1.5A peak

V

CC2

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .16V/-0.3V . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .20mA DC/1.5A peak

SS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6V/-0.3V . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .-100µA

COMP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6V/-0.3V . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .200µA

VFB . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6V/-0.3V . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .-0.2µA

C

OFF

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6V/-0.3V . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .-0.2µA

V

FFB

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6V/-0.3V . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .-0.2µA

V

ID0

- V

ID4

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6V/-0.3V . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .-50µA

V

GATE(H)

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .16V/-0.3V . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .100mA DC/1.5A peak

V

GATE(L)

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .16V/-0.3V . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .100mA DC/1.5A peak

LGnd . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .0V . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .25mA

PGnd . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .0V . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .100mA DC/1.5A peak

Operating Junction Temperature, TJ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .0° to 150°C

Lead Temperature Soldering

Reflow (SMD styles only) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .60 sec. max above 183°C, 230°C peak

Storage Temperature Range, TS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .−65° to 150°C

ESD Susceptibility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2kV

PARAMETER TEST CONDITIONS MIN TYP MAX UNIT

CS5157

Electrical Characteristics:

0°C < TA< +70°C; 0°C < TJ< +85°C; 8V < V

CC1

< 14V; 5V < V

CC2

< 14V; DAC Code: V

ID4

= V

ID2

=

V

ID1

= V

ID0

= 1; V

ID3

= 0; CV

GATE(L)

and CV

GATE(H)

= 1nF; C

OFF

= 330pF; CSS= 0.1µF, unless otherwise specified.

Absolute Maximum Ratings

■ Error Amplifier

VFBBias Current VFB= 0V 0.3 1.0 µA
Open Loop Gain 1.25V < V

COMP

< 4V; Note 1 50 60 dB
Unity Gain Bandwidth Note 1 500 3000 kHz
COMP SINK Current V

COMP

= 1.5V; VFB= 3V; VSS> 2V 0.4 2.5 8.0 mA

COMP SOURCE Current V

COMP

= 1.2V; VFB= 2.7V; VSS= 5V 30 50 70 µA

COMP CLAMP Current V

COMP

= 0V; VFB= 2.7V 0.4 1.0 1.6 mA
COMP High Voltage VFB= 2.7V; VSS= 5V 4.0 4.3 5.0 V
COMP Low Voltage VFB=3V 160 600 mV
PSRR 8V < V

CC1

< 14V @ 1kHz; Note 1 60 85 dB

■ V

CC1

Monitor

Start Threshold Output switching 3.75 3.90 4.05 V
Stop Threshold Output not switching 3.70 3.85 4.00 V
Hysteresis Start-Stop 50 mV

■ DAC

Input Threshold V

ID0

, V

ID1

, V

ID2

, V

ID3

, V

ID4

1.00 1.25 2.40 V

Input Pull Up Resistance V

ID0

, V

ID1

, V

ID2

, V

ID3

, V

ID4

25 50 100 kΩ
Pull Up Voltage 4.85 5.00 5.15 V
Accuracy Measure VFB= V

COMP

, 25°C ≤ TJ≤ 85°C 1.0 %

V

ID4VID3VID2VID1VID0

0 1 1 1 1 1.2870 1.3000 1.3130 V
0 1 1 1 0 1.3365 1.3500 1.3635 V
0 1 1 0 1 1.3860 1.4000 1.4140 V
0 1 1 0 0 1.4355 1.4500 1.4645 V
0 1 0 1 1 1.4850 1.5000 1.5150 V

0 1 0 1 0 1.5345 1.5500 1.5655 V
0 1 0 0 1 1.5840 1.6000 1.6160 V

0 1 0 0 0 1.6335 1.6500 1.6665 V
0 0 1 1 1 1.6830 1.7000 1.7170 V

0 0 1 1 0 1.7325 1.7500 1.7675 V
0 0 1 0 1 1.7820 1.8000 1.8180 V

CS5157

3

PARAMETER TEST CONDITIONS MIN TYP MAX UNIT

Electrical Characteristics:

0°C < TA< +70°C; 0°C < TJ< +85°C; 8V < V

CC1

< 14V; 5V < V

CC2

< 14V; DAC Code: V

ID4

= V

ID2

=

V

ID1

= V

ID0

= 1; V

ID3

= 0; CV

GATE(L)

and CV

GATE(H)

= 1nF; C

OFF

= 330pF; CSS= 0.1µF, unless otherwise specified.

■ DAC: continued

V

ID4VID3VID2VID1VID0

0 0 1 0 0 1.8315 1.8500 1.8685 V
0 0 0 1 1 1.8810 1.9000 1.9190 V

0 0 0 1 0 1.9305 1.9500 1.9695 V
0 0 0 0 1 1.9800 2.0000 2.0200 V
0 0 0 0 0 2.0295 2.0500 2.0705 V

1 1 1 1 1 1.2315 1.2440 1.2564 V
1 1 1 1 0 2.0790 2.1000 2.1210 V
1 1 1 0 1 2.1780 2.2000 2.2220 V
1 1 1 0 0 2.2770 2.3000 2.3230 V
1 1 0 1 1 2.3760 2.4000 2.4240 V
1 1 0 1 0 2.4750 2.5000 2.5250 V
1 1 0 0 1 2.5740 2.6000 2.6260 V
1 1 0 0 0 2.6730 2.7000 2.7270 V
1 0 1 1 1 2.7720 2.8000 2.8280 V

1 0 1 1 0 2.8710 2.9000 2.9290 V
1 0 1 0 1 2.9700 3.0000 3.0300 V
1 0 1 0 0 3.0690 3.1000 3.1310 V
1 0 0 1 1 3.1680 3.2000 3.2320 V
1 0 0 1 0 3.2670 3.3000 3.3330 V
1 0 0 0 1 3.3660 3.4000 3.4340 V
1 0 0 0 0 3.4650 3.5000 3.5350 V

■ V

GATE(H)

and V

GATE(L)

Out SOURCE Sat at 100mA Measure V

CC1

– V

GATE(L),;VCC2

– V

GATE(H)

1.2 2.0 V

Out SINK Sat at 100mA Measure V

GATE(H)

– V

PGnd

; 1.0 1.5 V

V

GATE(L)

– V

PGnd

Out Rise Time 1V < V

GATE(H)

< 9V; 1V < V

GATE(L)

< 9V 30 50 ns

V

CC1

= V

CC2

= 12V

Out Fall Time 9V > V

GATE(H)

> 1V; 9V > V

GATE(L)

> 1V 30 50 ns

V

CC1

= V

CC2

= 12V

Delay V

GATE(H)

to V

GATE(L)

V

GATE(H)

falling to 2V; V

CC1

= V

CC2

= 8V 25 50 ns

V

GATE(L)

rising to 2V

Delay V

GATE(L)

to V

GATE(H)

V

GATE(L)

falling to 2V; V

CC1

= V

CC2

= 8V 25 50 ns

V

GATE(H)

rising to 2V

V

GATE(H)

, V

GATE(L)

Resistance Resistor to LGnd 20 50 100 kΩ

V

GATE(H)

, V

GATE(L)

Schottky LGnd to V

GATE(H)

@ 10mA 600 800 mV

LGnd to V

GATE(L)

@ 10mA

■ Soft Start (SS)

Charge Time 1.6 3.3 5.0 ms
Pulse Period 25 100 200 ms
Duty Cycle (Charge Time/Pulse Period) × 100 1.0 3.3 6.0 %
COMP Clamp Voltage VFB= 0V; VSS= 0 0.50 0.95 1.10 V
V

FFB

SS Fault Disable V

GATE(H)

= Low; V

GATE(L)

= Low 0.9 1.0 1.1 V

High Threshold 2.5 3.0 V

PARAMETER TEST CONDITIONS MIN TYP MAX UNIT

4

CS5157

Package Pin Description

PACKAGE PIN # PIN SYMBOL FUNCTION

Electrical Characteristics:

0°C < TA< +70°C; 0°C < TJ< +85°C; 8V < V

CC1

< 14V; 5V < V

CC2

< 14V; DAC Code: V

ID4

= V

ID2

= V

ID1

= V

ID0

= 1; V

ID3

= 0; CV

GATE(L)

and CV

GATE(H)

= 1nF; C

OFF

= 330pF; CSS= 0.1µF, unless otherwise specified.

16L SO Narrow

1,2,3,4,6 V

ID0

– V

ID4

Voltage ID DAC input pins. These pins are internally pulled up to 5V
providing logic ones if left open. V

ID4

selects the DAC range. When V

ID4

is High (logic one), the DAC range is 2.10V to 3.50V with 100mV incre­ments. When V

ID4

is Low (logic zero), the DAC range is 1.30V to 2.05V

with 50mV increments. V

ID0

- V

ID4

select the desired DAC output volt­age. Leaving all 5 DAC input pins open results in a DAC output voltage
of 1.2440V, allowing for adjustable output voltage, using a traditional
resistor divider.

5 SS Soft Start Pin. A capacitor from this pin to LGnd in conjunction with

internal 60µA current source provides soft start function for the con­troller. This pin disables fault detect function during Soft Start. When a
fault is detected, the soft start capacitor is slowly discharged by internal
2µA current source setting the time out before trying to restart the IC.
Charge/discharge current ratio of 30 sets the duty cycle for the IC when
the regulator output is shorted.

7C

OFF

A capacitor from this pin to ground sets the time duration for the on
board one shot, which is used for the constant off time architecture.

8V

FFB

Fast feedback connection to the PWM comparator. This pin is connected
to the regulator output. The inner feedback loop terminates on time.

9V

CC2

Boosted power for the high side gate driver.

10 V

GATE(H)

High FET driver pin capable of 1.5A peak switching current. Internal cir­cuit prevents V

GATE(H)

and V

GATE(L)

from being in high state simultane-

ously.

■ PWM Comparator

Transient Response V

FFB

= 0 to 5V to V

GATE(H)

= 9V to 1V; 100 125 ns

V

CC1

= V

CC2

= 12V

V

FFB

Bias Current V

FFB

= 0V 0.3 µA

■ Supply Current

I

CC1

No Switching 8.5 13.5 mA

I

CC2

No Switching 1.6 3.0 mA

Operating I

CC1

VFB= COMP = V

FFB

813mA

Operating I

CC2

VFB= COMP = V

FFB

25mA

■ C

OFF

Normal Charge Time V

FFB

= 1.5V; VSS= 5V 1.0 1.6 2.2 µs

Extension Charge Time VSS= V

FFB

= 0 5.0 8.0 11.0 µs

Discharge Current C

OFF

to 5V; V

FB

>1V 5.0 mA

■ Time Out Timer

Time Out Time VFB= V

COMP

; V

FFB

= 2V; 10 30 50 µs

Record V

GATE(H)

Pulse High Duration

Fault Mode Duty Cycle V

FFB

= 0V 35 50 65 %

Note 1: Guaranteed by design, not 100% tested in production.

5

CS5157

Block Diagram

Package Pin Description: continued

PACKAGE PIN # PIN SYMBOL FUNCTION

16L SO Narrow

11 PGnd High current ground for the IC. The MOSFET drivers are referenced to

this pin. Input capacitor ground and the source of lower FET should be
tied to this pin.

12 V

GATE(L)

Low FET driver pin capable of 1.5A peak switching current.

13 V

CC1

Input power for the IC and low side gate driver.

14 LGnd Signal ground for the IC. All control circuits are referenced to this pin.

15 COMP Error amplifier compensation pin. A capacitor to ground should be

provided externally to compensate the amplifier.

16 V

FB

Error amplifier DC feedback input. This is the master voltage feedback
which sets the output voltage. This pin can be connected directly to the
output or a remote sense trace.

V

CC1

SS

V

ID0

V

ID1

V

ID2

V

ID3

V

ID4

V

FB

COMP

V

FFB

LGnd

V

CC1

Comparator

-

+

3.90V

3.85V

5 BIT

DAC

Slow Feedback

Fast Feedback

Monitor

Error
Amplifier

+

-

V

CC2

V

GATE(H)

V

GATE(L)

60µA

2µA

5V

2.5V

0.7V

-

+

+

-

SS Low
Comparator

SS High
Comparator

R

S

FAULT

Latch

FAULT

Q

FAULT

Q

PGnd

V

CC1

PGnd

PWM
Comparator

-

Off-Time

Timeout

R

S

PWM
Latch

Edge Triggered

+

-

+

V

Low

FFB

1V

Comparator

PWM
COMP

Maximum

On-Time
Timeout

Normal
Off-Time
Timeout

Extended

Off-Time
Timeout

Time Out

Timer

(30µs)

Q

Q

GATE(H) = ON

GATE(H) = OFF

C

OFF

One Shot

R

S

Q

C

OFF

V

2

TM

Control Method

The V

2

TM

method of control uses a ramp signal that is gen­erated by the ESR of the output capacitors. This ramp is
proportional to the AC current through the main inductor
and is offset by the value of the DC output voltage. This
control scheme inherently compensates for variation in
either line or load conditions, since the ramp signal is gen­erated from the output voltage itself. This control scheme
differs from traditional techniques such as voltage mode,
which generates an artificial ramp, and current mode,
which generates a ramp from inductor current.

Figure 1: V

2

TM

Control Diagram

The V

2

TM

control method is illustrated in Figure 1. The out­put voltage is used to generate both the error signal and the
ramp signal. Since the ramp signal is simply the output
voltage, it is affected by any change in the output regard­less of the origin of that change. The ramp signal also con­tains the DC portion of the output voltage, which allows
the control circuit to drive the main switch to 0% or 100%
duty cycle as required.

A change in line voltage changes the current ramp in the
inductor, affecting the ramp signal, which causes the V

2

TM

control scheme to compensate the duty cycle. Since the
change in inductor current modifies the ramp signal, as in
current mode control, the V

2

TM

control scheme has the same

advantages in line transient response.
A change in load current will have an affect on the output

voltage, altering the ramp signal. A load step immediately
changes the state of the comparator output, which controls
the main switch. Load transient response is determined
only by the comparator response time and the transition
speed of the main switch. The reaction time to an output
load step has no relation to the crossover frequency of the
error signal loop, as in traditional control methods.

The error signal loop can have a low crossover frequency,
since transient response is handled by the ramp signal loop.
The main purpose of this ‘slow’ feedback loop is to provide
DC accuracy. Noise immunity is significantly improved,
since the error amplifier bandwidth can be rolled off at a low
frequency. Enhanced noise immunity improves remote sens­ing of the output voltage, since the noise associated with
long feedback traces can be effectively filtered.

Line and load regulation are drastically improved because
there are two independent voltage loops. A voltage mode
controller relies on a change in the error signal to compen-

sate for a deviation in either line or load voltage. This
change in the error signal causes the output voltage to
change corresponding to the gain of the error amplifier,
which is normally specified as line and load regulation. A
current mode controller maintains fixed error signal under
deviation in the line voltage, since the slope of the ramp
signal changes, but still relies on a change in the error sig­nal for a deviation in load. The V

2

TM

method of control
maintains a fixed error signal for both line and load varia­tion, since the ramp signal is affected by both line and load.

Constant Off Time

To maximize transient response, the CS5157 uses a constant
off time method to control the rate of output pulses. During
normal operation, the off time of the high side switch is ter­minated after a fixed period, set by the C

OFF

capacitor. To

maintain regulation, the V

2

TM

control loop varies switch on
time. The PWM comparator monitors the output voltage
ramp, and terminates the switch on time.

Constant off time provides a number of advantages. Switch
duty cycle can be adjusted from 0 to 100% on a pulse by
pulse basis when responding to transient conditions. Both
0% and 100% duty cycle operation can be maintained for
extended periods of time in response to load or line tran­sients. PWM slope compensation to avoid sub-harmonic
oscillations at high duty cycles is avoided.

Switch on time is limited by an internal 30µs timer, mini­mizing stress to the power components.

Programmable Output

The CS5157 is designed to provide two methods for pro­gramming the output voltage of the power supply. A five
bit on board digital to analog converter (DAC) is used to
program the output voltage within two different ranges.
The first range is 2.10V to 3.50V in 100mV steps, the second
is 1.30V to 2.05V in 50mV steps, depending on the digital
input code. If all five bits are left open, the CS5157 enters
adjust mode. In adjust mode, the designer can choose any
output voltage by using resistor divider feedback to the
VFBand V

FFB

pins, as in traditional controllers.

Start Up

Until the voltage on the V

CC1

supply pin exceeds the 3.9V
monitor threshold, the soft start and gate pins are held low.
The FAULT latch is reset (no Fault condition). The output
of the error amplifier (COMP) is pulled up to 1V by the
comparator clamp. When the V

CC1

pin exceeds the monitor
threshold, the GateH output is activated, and the soft start
capacitor begins charging. The GateH output will remain
on, enabling the NFET switch, until terminated by either
the PWM comparator, or the maximum on time timer.

If the maximum on time is exceeded before the regulator
output voltage achieves the 1V level, the pulse is terminat­ed. The GateH pin drives low, and the GateL pin drives
high for the duration of the extended off time. This time is
set by the time out timer and is approximately equal to the
maximum on time, resulting in a 50% duty cycle. The
GateL pin will then drive low, the GateH pin will drive
high, and the cycle repeats.

When regulator output voltage achieves the 1V level pre­sent at the COMP pin, regulation has been achieved and

Theory of Operation

CS5157

Applications Information

6

COMP

PWM
Comparator

+

C

–

Ramp
Signal

Error

Signal

Error

Amplifier

V

GATE(H)

V

GATE(L)

V

FFB

Output
Voltage
Feedback

V

FB

–

E

+

Reference

Voltage

CS5157

7

Applications Information: continued

normal off time will ensue. The PWM comparator termi­nates the switch on time, with off time set by the C

OFF

capacitor. The V

2

TM

control loop will adjust switch duty
cycle as required to ensure the regulator output voltage
tracks the output of the error amplifier.

The soft start and COMP capacitors will charge to their
final levels, providing a controlled turn on of the regulator
output. Regulator turn on time is determined by the COMP
capacitor charging to its final value. Its voltage is limited
by the soft start COMP clamp and the voltage on the soft
start pin (see Figures 2 and 3).

Figure 2: CS5157 demonstration board startup in response to increasing
12V and 5V input voltages. Extended off time is followed by normal off
time operation when output voltage achieves regulation to the error
amplifier output.

Figure 3: CS5157 demonstration board startup waveforms.

If the input voltage rises quickly, or the regulator output is
enabled externally, output voltage will increase to the level
set by the error amplifier output more rapidly, usually
within a couple of cycles (see Figure 4).

Figure 4: CS5157 demonstration board enable startup waveforms.

Normal Operation

During normal operation, switch off time is constant and
set by the C

OFF

capacitor. Switch on time is adjusted by the

V

2

TM

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 rip­ple current working into the ESR of the output capacitors
(see Figures 5 and 6).

Figure 5: Peak-to-peak ripple on V

OUT

= 2.8V, I

OUT

= 0.5A (light load).

Trace 1 - Regulator Output Voltage (1V/div.)
Trace 2 - Inductor Switching Node (2V/div.)
Trace 3 - 12V input (V
Trace 4 - 5V Input (1V/div.)

CC1

and V

CC2

) (5V/div.)

Trace 1 - Regulator Output Voltage (5V/div.)
Trace 2 - Inductor Switching Node (5V/div.)

Ch1 High

2.80V

Trace 1 - Regulator Output Voltage (1V/div.)
Trace 3 - COMP Pin (error amplifier output) (1V/div.)
Trace 4 - Soft Start Pin (2V/div.)

Trace 1 - Regulator Output Voltage (10mV/div.)
Trace 2 - Inductor Switching Node (5V/div.)

Applications Information: continued

CS5157

8

Figure 6: Peak-to-peak ripple on V

OUT

= 2.8V, I

OUT

= 13A (heavy load).

Transient Response

The CS5157 V

2

TM

control loop’s 100ns reaction time pro­vides 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 current
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.

For best transient response, a combination of a number of
high frequency and bulk output capacitors are usually
used.

If the maximum on time is exceeded while responding to a
sudden increase in load current, a normal off time occurs to
prevent saturation of the output inductor.

Figure 7: CS5157 demonstration board response to a 0.5 to 13A load
pulse (output set for 2.8V).

Figure 8: CS5157 demonstration board response to 13A load turn on
(output set for 2.8V). Upon completing a normal off time, the V

2

TM

con­trol loop immediately connects the inductor to the input voltage, pro­viding 100% duty cycle. Regulation is achieved in less than 20µs.

Figure 9: CS5157 demonstration board response to 13A load turn off
(output set for 2.8V). V

2

TM

control topology immediately connects
inductor to ground, providing 0% duty cycle. Regulation is achieved in
less than 10µs.

V

CC1

Monitor

To maintain predictable startup and shutdown characteris­tics an internal V

CC1

monitor circuit is used to prevent the
part from operating below 3.75V minimum startup. The
V

CC1

monitor comparator provides hysteresis and guaran-

tees a 3.70V minimum shutdown threshold.

Protection and Monitoring Features

Trace1 - Regulator Output Voltage (10mV/div.)
Trace 2 - Inductor Switching Node (5V/div.)

Trace 1 - Regulator Output Voltage (100mV/div.)
Trace 2 - Inductor Switching Node (5V/div.)
Trace 3 - Output Current (0.5 to 13 Amps) (10A/div.)

Trace1 - Regulator Output Voltage (100mV/div.)
Trace 2 - Inductor Switching Node (5V/div.)
Trace 3 - Output Current (13 to 0.5 Amps) (10A/div.)

Trace 1 - Regulator Output Voltage (100mV/div.)
Trace 3 - Regulator Output Current (10A/div.)

CS5157

Applications Information: continued

9

Short Circuit Protection

A lossless hiccup mode short circuit protection feature is
provided, requiring only the soft start capacitor to imple­ment. If a short circuit condition occurs (V

FFB

< 1V), the V

FFB

low comparator sets the FAULT latch. This causes the top
MOSFET to shut off, disconnecting the regulator from it’s
input voltage. The soft start capacitor is then slowly dis­charged by a 2µA current source until it reaches it’s lower

0.7V threshold. The regulator will then attempt to restart nor­mally, operating in it’s extended off time mode with a 50%
duty cycle, while the soft start capacitor is charged with a
60µA charge current.

If the short circuit condition persists, the regulator output
will not achieve the 1V low V

FFB

comparator threshold
before the soft start capacitor is charged to it’s upper 2.5V
threshold. If this happens the cycle will repeat itself until the
short is removed. The soft start charge/discharge current
ratio sets the duty cycle for the pulses (2µA/60µA = 3.3%),
while actual duty cycle is half that due to the extended off
time mode (1.65%).

This protection feature results in less stress to the regulator
components, input power supply, and PC board traces
than occurs with constant current limit protection (see
Figures 10 and 11).

If the short circuit condition is removed, output voltage
will rise above the 1V level, preventing the FAULT latch
from being set, allowing normal operation to resume.

Figure 10: CS5157 demonstration board hiccup mode short circuit pro­tection. Gate pulses are delivered while the soft start capacitor charges,
and cease during discharge.

Figure 11: Startup with regulator output shorted.

Overvoltage Protection

Overvoltage protection (OVP) is provided as result of the
normal operation of the V

2

TM

control topology and requires
no additional external components. The control loop
responds to an overvoltage condition within 100ns, causing
the top MOSFET to shut off, disconnecting the regulator
from it’s input voltage. The bottom MOSFET is then acti­vated, resulting in a “crowbar” action to clamp the output
voltage and prevent damage to the load (see Figures 12 and

13). The regulator will remain in this state until the over­voltage condition ceases or the input voltage is pulled low.
The bottom FET and board trace must be properly
designed to implement the OVP function.

Figure 12: OVP response to an input-to-output short circuit by immedi­ately providing 0% duty cycle, crow-barring the input voltage to
ground.

Trace 4 = 5V from PC Power Supply (2V/div.)
Trace 2 = Inductor Switching Node (2V/div.)

Trace 4 - 5V Supply Voltage (2V/div.)
Trace 3 - Soft Start Timing Capacitor (1V/div.)
Trace 2 - Inductor Switching Node (2V/div.)

Trace 4 = 5V from PC Power Supply (5V/div.)
Trace1 = Regulator Output Voltage (1V/div.)
Trace 2 = Inductor Switching Node (5V/div.)

Applications Information: continued

CS5157

10

Figure 13: OVP response to an input-to-output short circuit by pulling
the input voltage to ground.

External Output Enable Circuit

On/off control of the regulator can be implemented
through the addition of two additional discrete compo­nents (see Figure 14). This circuit operates by pulling the
soft start pin high, and the V

FFB

pin low, emulating a short

circuit condition.

Figure 14: Implementing shutdown with the CS5157.

External Power Good Circuit

An optional Power Good signal can be generated through
the use of four additional external components (see Figure

15). The threshold voltage of the Power Good signal can be
adjusted per the following equation:

V

Power Good

=

This circuit provides an open collector output that drives
the Power Good output to ground for regulator voltages
less than V

Power Good

.

Figure 15: Implementing Power Good with the CS5157.

Figure 16: CS5157 demonstration board during power up. Power Good
signal is activated when output voltage reaches 1.70V.

Selecting External Components

The CS5157 can be used with a wide range of external
power components to optimize the cost and performance of
a particular design. The following information can be used
as general guidelines to assist in their selection.

NFET Power Transistors

Both logic level and standard MOSFETs can be used. The
reference designs derive gate drive from the 12V supply
which is generally available in most computer systems and
utilize logic level MOSFETs. Multiple MOSFETs may be
paralleled to reduce losses and improve efficiency and ther­mal management.

Voltage applied to the MOSFET gates depends on the
application circuit used. Both upper and lower gate driver
outputs are specified to drive to within 1.5V of ground
when in the low state and to within 2V of their respective
bias supplies when in the high state. In practice, the MOS­FET gates will be driven rail to rail due to overshoot caused
by the capacitive load they present to the controller IC. For
the typical application where V

CC1

= V

CC2

= 12V and 5V is

(R1 + R2) × 0.65V

R2

V

OUT

CS5157

R1

10k

R2

6.2k

5V

R3
10k

PN3904

Power Good

PN3904

Trace 4 = 5V from PC Power Supply (2V/div.)
Trace 1 = Regulator Output Voltage (1V/div.)

Shutdown

Input

5V

MMUN2111T1 (SOT-23)

5

SS

CS5157

8

V

FFB

IN4148

Ch 1 High

2.80V

Trace 3 = 12V Input (V
Trace 4 = 5V Input (2V/div.)
Trace 1 = Regulator Output Voltage (1V/div.)
Trace 2 = Power Good Signal (2V/div.)

CC1

) and V

) (10V/div.)

CC2

CS5157

Applications Information: continued

11

used as the source for the regulator output current, the fol­lowing gate drive is provided;

V

GATE(H)

= 12V - 5V = 7V, V

GATE(L)

= 12V (see Figure 17).

Figure 17: CS5157 gate drive waveforms depicting rail to rail swing.

The most important aspect of MOSFET performance is
RDSON, which effects regulator efficiency and MOSFET
thermal management requirements.

The power dissipated by the MOSFETs may be estimated
as follows;

Switching MOSFET:

Power = I

LOAD

2

× RDSON× duty cycle

Synchronous MOSFET:

Power = I

LOAD

2

× RDSON× (1 - duty cycle)

Duty Cycle =

Off Time Capacitor (C

OFF

)

The C

OFF

timing capacitor sets the regulator off time:

T

OFF

= C

OFF

× 4848.5

When the V

FFB

pin is less than 1V, the current charging the

C

OFF

capacitor is reduced. The extended off time can be cal-

culated as follows:

T

OFF

= C

OFF

× 24,242.5.

Off time will be determined by either the T

OFF

time, or the

time out timer, whichever is longer.
The preceding equations for duty cycle can also be used to

calculate the regulator switching frequency and select the
C

OFF

timing capacitor:

C

OFF

= ,

where:

Period =

Schottky Diode for Synchronous MOSFET

A Schottky diode may be placed in parallel with the syn­chronous MOSFET to conduct the inductor current upon
turn off of the switching MOSFET to improve efficiency.
The CS5157 reference circuit does not use this device due to
it’s excellent design. Instead, the body diode of the syn­chronous MOSFET is utilized to reduce cost and conducts
the inductor current. For a design operating at 200kHz or so,
the low non-overlap time combined with Schottky forward
recovery time may make the benefits of this device not
worth the additional expense (see Figure 6, channel 2). The
power dissipation in the synchronous MOSFET due to body
diode conduction can be estimated by the following equation:

Power = V

bd

× I

LOAD

× conduction time × switching frequency

Where Vbd= the forward drop of the MOSFET body diode.
For the CS5157 demonstration board as shown in Figure 6;

Power = 1.6V × 13A × 100ns × 233kHz = 0.48W

This is only 1.3% of the 36.4W being delivered to the load.

Input and Output Capacitors

These components must be selected and placed carefully to
yield optimal results. Capacitors should be chosen to pro­vide acceptable ripple on the input supply lines and regula­tor output voltage. Key specifications for input capacitors
are their ripple rating, while ESR is important for output
capacitors. For best transient response, a combination of
low value/high frequency and bulk capacitors placed close
to the load will be required.

Output Inductor

The inductor should be selected based on its inductance,
current capability, and DC resistance. Increasing the induc­tor value will decrease output voltage ripple, but degrade
transient response.

Thermal Considerations for Power MOSFETs and Diodes

In order to maintain good reliability, the junction tempera­ture of the semiconductor components should be kept to a
maximum of 150°C or lower. The thermal impedance (junc­tion to ambient) required to meet this requirement can be
calculated as follows:

Thermal Impedance =

T

JUNCTION(MAX)

- T

AMBIENT

Power

Thermal Management

1

switching frequency

Period × (1 - duty cycle)

4848.5

V

OUT

+ (I

LOAD

× RDS

ON OF SYNCH FET

)

VIN+ (I

LOAD

× RDS

ON OF SYNCH FET

) - (I

LOAD

× RDS

ON OF SWITCH FET

)

Trace 3 = V
Math 1= V
Trace 4 = V
Trace 2 = Inductor Switching Node (5V/div.)

GATE(H)

GATE(H)

GATE(L)

(10V/div.)

- 5V

IN

(10V/div.)

Applications Information: continued

CS5157

12

A heatsink may be added to TO-220 components to reduce
their thermal impedance. A number of PC board layout
techniques such as thermal bias and additional copper foil
area can be used to improve the power handling capability
of surface mount components.

EMI Management

As a consequence of large currents being turned on and off
at high frequency, switching regulators generate noise as a
consequence of their normal operation. When designing for
compliance with EMI/EMC regulations, additional compo­nents may be added to reduce noise emissions. These com­ponents are not required for regulator operation and exper­imental results may allow them to be eliminated. The input
filter inductor may not be required because bulk filter and
bypass capacitors, as well as other loads located on the
board will tend to reduce regulator di/dt effects on the cir­cuit board and input power supply. Placement of the
power component to minimize routing distance will also
help to reduce emissions.

Figure 18: Filter components Figure 19: Input Filter

Layout Guidelines

1. Place 12V filter capacitor next to the IC and connect
capacitor ground to pin 11 (PGnd).

2. Connect pin 11 (PGnd) with a separate trace to the
ground terminals of the 5V input capacitors.

3. Place fast feedback filter capacitor next to pin 8 (V

FFB

)
and connect it’s ground terminal with a separate, wide
trace directly to pin 14 (LGnd).

4. Connect the ground terminals of the Compensation
capacitor directly to the ground of the fast feedback filter
capacitor to prevent common mode noise from effecting
the PWM comparator.

5. Place the output filter capacitor(s) as close to the load as
possible and connect the ground terminal to pin 14 (LGnd).

6. Connect the V

FB

pin directly to the load with a separate

trace (remote sense).

7. Place 5V input capacitors close to the switching MOSFET
and synchronous MOSFET.

Route gate drive signals V

GATE(H)

(pin 10) and V

GATE(L)

(pin 12 when used) with traces that are a minimum of 0.025
inches wide.

Figure 20: Layout Guidelines

33Ω

2µH

1200µF x 3/16V

2µH

+

1.0µF
V

COMP

VCC

0.1µF

SOFTSTART

To the negative terminal of the output capacitors

To the negative terminal of the

15

5

11

8

OFF TIME

input capacitors

100pF
V

FFB

1000pF

CS5157

13

Figure 21: 5V to 3.3V/10A converter.

Figure 22: 5V to 3.3V/10A converter with current sharing.

Figure 23: 3.3V to 2.5V/7A converter with 12V bias.

MBRS

Additional Application Circuits

120

0.1µF

0.1µF

330pF

MBRS120

1µF

0.33µF

5V

V

CC1

V

ID0

V

ID1

V

ID2

V

ID3

V

ID4

C

OFF

SS

COMP

MBRS120

V

CC2

CS5157

LGnd

V

GATE(H)

V

GATE(L)

PGnd

V

V

12V

1µF

+

Si9410DY

1200µF/10V x 3
AIEI

3µH

+

3.3V/10A

1200µF/10V x 5
AIEI

0.1µF

330pF

0.33µF

V

CC1

V

ID0

V

ID1

V

ID2

V

ID3

V

ID4

C

OFF

SS

COMP

V

CC2

CS5157

LGnd

V

GATE(H)

V

GATE(L)

V

PGnd

V

FB

FFB

1µF

Si4410DY

FB

FFB

3.3k

100pF

Si9410

100pF

3.3k

3.3V

Si9410

+

1200µF/10V x 3
AIEI

5µH

+

1200µF/10V x 5
AIEI

2.5V/7A

5V

MBRS

120

0.1µF

MBRS120

1µF

330pF

0.1µF

0.33µF

V

CC1

V

ID0

V

ID1

V

ID2

V

ID3

V

ID4

C

OFF

SS

COMP

MBRS120

V

CC2

CS5157

LGnd

V

GATE(H)

V

GATE(L)

V

PGnd

V

1µF

Si4410

FB

FFB

3.3k

100pF

+

Si9410

1200µF/10V x 3
AIEI

3µH

10Ω

Remote

Sense

3.3V/10A

1200µF/10V x 5

+

AIEI

Connect to
other circuits for
current sharing

14

© 1999 Cherry Semiconductor Corporation

Cherry Semiconductor Corporation reserves the right to
make changes to the specifications without notice. Please
contact Cherry Semiconductor Corporation for the latest
available information.

Thermal Data 16L

SO Narrow

R

ΘJC

typ 28 ˚C/W

R

ΘJA

typ 115 ˚C/W

D

Lead Count Metric English

Max Min Max Min

16L SO Narrow 10.00 9.80 .394 .386

CS5157

Package Specification

PACKAGE DIMENSIONS IN mm (INCHES)

PACKAGE THERMAL DATA

Rev. 12/31/98

Ordering Information

Part Number Description

CS5157GD16 16L SO Narrow
CS5157GDR16 16L SO Narrow (tape & reel)

Surface Mount Narrow Body (D); 150 mil wide

1.27 (.050) BSC

0.51 (.020)

0.33 (.013)

6.20 (.244)

5.80 (.228)

4.00 (.157)

3.80 (.150)

1.57 (.062)

1.37 (.054)

D

0.25 (0.10)

0.10 (.004)

1.75 (.069) MAX

1.27 (.050)

0.40 (.016)

REF: JEDEC MS-012

0.25 (.010)

0.19 (.008)